EXCHANGE 


The  Adsorption  of  Sulfur  Dioxide  by  the 
Gel  of  Silicic  Acid 


A  DISSERTATION 


SUBMITTED  TO  THE  BOARD  OF  UNIVERSITY  STUDIES  OF  THE 

JOHNS  HOPKINS  UNIVERSITY  IN  PARTIAL 'FULFILMENT 

OF  THE  REQUIREMENTS  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


BY 

JOHN  McGAVACK,  JR. 

BALTIMORE,  MD. 

February,  1920 


EASTON,  PA.:. 

ESCHENBACH  PRINTING  COMPANY 
1920 


The  Adsorption  of  Sulfur  Dioxide  by  the 
Gel  of  Silicic  Acid 


A  DISSERTATION 


SUBMITTED  TO  THE  BOARD  OF  UNIVERSITY  STUDIES  OF  THE 

JOHNS  HOPKINS  UNIVERSITY  IN  PARTIAL  FULFILMENT 

OF  THE  REQUIREMENTS  FOR  THE  DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


BY 

JOHN  McGAVACK,  JR. 

BALTIMORE,  MD. 

February,  1920 


EASTON,  PA.: 

ESCHENBACH  PRINTING  COMPANY 
1920 


CONTENTS. 

Pa&e. 

Introduction 5 

Apparatus 7 

Fig.  1 7 

Materials 9 

Procedure , 12 

Fig.  2 12 

Fig.  3 :  •  •  13 

Experimental 13 

Water  Content  and  Adsorption 17 

Fig.  4 18 

Adsorption  Reversible 20 

Fig.  5 22 

Fig.  6 ; 24 

Fig.  7 25 

Discussion 26 

Fig.  8 32 

Fig.  9 33 

Fig.  10 , 36 

Summary 37 


46200i: 


ACKNOWLEDGMENT. 

This  investigation  was  carried  out  under  the  advice  and  with  the  as- 
sistance of  Professor  Patrick.  I  wish  to  take  this  opportunity  to  express 
to  him  my  sincere  appreciation  of  the  help  which  he  gave.  I  also  feel 
under  obligation  to  Professors  Frazer,  Lovelace,  Reid  and  Gilpin  and  to 
Dr.  Thornton  for  instruction  and  encouragement  received  from  them. 

I  also  wish  to  take  this  opportunity  to  thank  Mr.  W.  T.  Levitt  for  his 
aid  in  the  construction  of  the  apparatus  used. 


The  Adsorption  of  Sulfur  Dioxide 
by  the  Gel  of  Silicic  Acid. 


Introduction. 

Many  investigations  of  the  adsorption  of  vapors  by  porous  bodies 
have  been  made  without  a  satisfactory  explanation  of  the  phenomenon 
being  found.  The  fact  that  the  adsorbing  material  is  not  chemically 
definite  but  has  adsorbent  properties  dependent  upon  its  method  of  prepara- 
tion is  not  the  least  of  the  reasons  for  apparent  lack  of  agreement  both  in 
experimental  results  and  theoretical  conclusions.  Again,  the  possibility, 
and  in  many  cases,  the  great  probability  of  chemical  reaction  occurring 
during  the  process  bring  in  another  factor  which  is  hard  to  control. 

In  the  investigations  carried  on  in  this  laboratory  during  the  war  it 
was  found  necessary  to  test  many  types  of  adsorbents,  both  as  to  their 
specific  action  against  poisonous  war  gases  as  well  as  to  their  suscepti- 
bility towards  other  vapors  and  gases.  It  was  realized  in  the  beginning 
that  porous  bodies — mere  mechanical  condensers  so  to  speak — were 
going  to  play  an  important  part.  Charcoal  was  brought  into  use  and 
its  protective  ability  greatly  increased  by  improved  methods  of  prepara- 
tion. This  laboratory  focused  a  good  part  of  its  attention  upon  colloidal 
substances  and  gels.  The  gel  of  silicic  acid,  having  been  previously 
shown  to  possess  adsorptive  properties,  received  first  attention.  The 
main  difficulty  was  its  large  scale  preparation.  Up  to  this  time  the  method 


of  dialysis,  a  long  and  tedious  process  had  been  used.  This  difficulty 
was  overcome  and  a  quick  and  simple  method,  of  which  more  will  be  said 
later,  was  developed.  A  product  of  high  adsorptive  power  resulted. 

This  gel  is  a  hard,  translucent,  porous  solid,  chemically  inert  and  with 
proper  precautions  can  be  reproduced  with  definite  exactness.  Hence 
it  is  an  ideal  substance  by  which  the  2  objectionable  features  mentioned 
above  might  be  eliminated.  It  is  true  that  it  always  contains  a  certain 
amount  of  water,  either  combined  or  adsorbed,  but  this  factor  may  be 
kept  constant  and  thus  will  not  interfere  with  the  more  important  in- 
vestigation. 

Thomas  Graham1  gives  the  first  account  of  the  preparation  of  silicic 
acid  gel  and  the  fact  that  it  possesses  a  power  of  adsorption  has  been 
known  since  that  time.  Nevertheless,  it  was  not  until  25  years  later, 
when  van  Bemmelen2  commenced  his  lengthy  and  important  experiments, 
that  this  property  was  investigated  more  thoroughly.  This  author  made 
an  exhaustive  study  of  the  hydration  and  dehydration  of  the  gel  in  all 
cases,  showing  that  these  two  curves  did  not  follow  the  same  path.  This 
hysteresis  will  be  taken  up  further  on  in  the  paper. 

Zsigmondy3  became  interested  in  this  substance  and  has  published 
several  articles  on  its  structure,  data  for  which  were  obtained  chiefly 
from  ultramicroscopic  investigations. 

Anderson,4  working  in  Zsigmondy's  laboratory,  studied  the  systems, 
gel-water,  gel-alcohol,  gel-benzene.  That  is,  he  determined  the  equi- 
librium weight  of  each  substance  adsorbed  per  gram  of  gel  at  points  corre- 
sponding to  different  pressures  of  the  material  adsorbed.  Like  that  of 
van  Bemmelen,  the  curve  obtained  by  emptying  the  pores  did  not  coin- 
cide with  that  observed  when  they  were  being  filled,  although  the  differ- 
ence between  the  2  paths  was  by  no  means  as  great  as  in  the  earlier  work. 
It  may  also  be  mentioned  that  while  van  Bemmelen  worked  entirely  under 
normal  atmospheric  pressure  Anderson,  on  the  other  hand,  did  his  work 
under  a  vacuum  produced  by  the  means  of  a  high  grade  oil  pump. 

Patrick5  was  the  first  investigator  of  gas  adsorption  by  this  substance. 
He  measured  the  amount  of  carbon  dioxide,  sulfur  dioxide  and  ammonia 
adsorbed  by  this  gel  at  different  pressures  for  a  number  of  different  tem- 
peratures. He  did  not  attempt  to  study  the  reverse  adsorption  path, 
nor  did  he  use  samples  of  the  material  containing  different  water  content. 

1  T.  Graham,  Phil.  Trans.,  151,  183-224  (1861);  also  Ann.,  121,  1-77  (1862); 
Proc.  Roy.  Soc.,  1864. 

2  J.  M.  van  Bemmelen,  Z.  anorg.  Chem.,  13,  233-356  (1896);  "Die  Adsorption,'1 
p.  196  (1910). 

3  Zsigmondy,  Z.  anorg.  Chem.,  71,  356  (1911). 

4  Anderson,  "Inaugural  Dissertation,"  Gottingen,  1914. 

B  W.  A.  Patrick,  "Inaugural  Dissertation,"  Goettingen,   1914.     See  also  Koll.  Z., 
13-14. 


The  purpose  of  the  present  work  may  now  be  stated  more  clearly: 
to  investigate  the  effect  of  different  water  content  of  the  gel  upon  its  ad- 
sorptive  powers;  to  obtain  measurements  where  temperature  control 
and  complete  exhaustion  could  be  more  rigidly  maintained  than  hereto- 
fore; and  by  using  an  inert  body  to  interpret,  if  possible,  the  mechanism 
by  which  this  phenomenon  adsorption  occurs. 

Apparatus. 

The  apparatus  used  in  these  measurements  is  shown  in  Fig.  i.  In 
general  outline  it  is  similar  to  that  used  by  Homfray1  in  her  work  on  char- 
coal and  later  by  Patrick,  in  the  original  investigation  of  gas  adsorption 
by  silica  gel.  The  essential  parts  are  the  gas  container  A,  the  gas  buret 
B,  the  adsorption  bulb  C  and  the  manometer  D.  These  parts  were  all 
sealed  together  and  mounted  inside  of  a  constant  temperature  bath  about 
which  more  will  be  said  later.  The  gas  container  was  a  steel  cylinder  filled 


with  liquid  sulfur  dioxide,  the  outlet  of  which  was  controlled  by  a  sensitive 
valve.  This  was  connected  to  the  gas  buret  by  means  of  drying  tubes 
a  and  a',  containing  calcium  chloride  and  phosphorus  pentoxide,  respec- 
tively, and  the  3 -way  mercury  stopcock  b.  In  order  to  fill  the  buret  the 
stopcock  b  was  opened  to  the  adsorption  apparatus  and  the  mercury 
bulb  c  raised  until  all  the  air  was  forced  out  of  the  buret.  The  cock  was 
then  opened  to  the  gas  container  and  the  mercury  well  c  was  lowered. 
1  Z.  pJiys.  Chem.,  74,  129  (1910). 


8 

Opening  the  cock  b  to  the  adsorption  part  of  the  mercury  was  again  raised. 
This  operation,  repeated  several  times,  removed  a  larger  part  of  the  air. 
To  remove  the  last  traces  of  air  the  bulb  was  lowered  just  so  the  mercury 
stood  at  the  level  d  in  the  buret.  The  cock  b  was  then  opened  to  the  gas 
container  A  and  sulfur  dioxide  was  allowed  to  sweep  out  the  whole  system 
for  a  considerable  period  of  time.  The  exit  tube  from  b  was  also  swept 
out  in  a  similar  manner. 

The  gas  buret  B  consisted  of  a  graduated  pipet  connected  by  a  U- joint 
to  another  tube,  e,  of  the  same  bore,  which  served  as  an  open  manometer. 
This  buret  was  recalibrated,  mercury  being  used  and  the  operation  being 
carried  out  in  a  30°  constant  temperature  bath.  As  with  most  gases,  all 
gas  volumes  were  measured  at  this  temperature,  and  if  the  temperature 
difference  was  less  than  10°,  no  correction  for  glass  expansion  was  deemed 
necessary.  To  determine  the  amount  of  gas  introduced,  the  mercury  in 
the  two  arms  of  the  buret  was  leveled,  this  balance  being  adjusted  by 
means  of  a  very  sensitive  gear  arrangement  which  enabled  the  reservoir 
C  to  be  raised  or  lowered  a  small  fraction  of  a  millimeter,  the  correct  posi- 
tion being  ascertained  by  means  of  the  cathetometer  telescope.  The 
reading  of  the  cathetometer  vernier,  calibrated  directly  into  o.oi  mm. 
divisions,  was  then  taken.  In  like  manner  another  reading  was  made 
after  the  gas  introduction.  By  reference  to  the  calibration  curve  these 
readings  were  transferred  into  cc.  and  were  then  corrected  to  standard 
conditions,  760  mm.  and  o°.  As  the  height  of  one  mm.  was  equivalent 
to  0.19  cc.  and  as  duplicate  settings  of  the  cathetometer  could  be  made 
within  0.03  mm.  the  maximum  error  in  reading  gas  volumes  was  0.005 
cc.  As  the  adsorption  proved  to  be  considerable  the  cc.  readings  are 
given  only  to  the  second  decimal  place. 

The  gas  buret  was  connected  by  a  glass  tube  of  small  bore  to  the  3-way 
stopcock  g,  which  in  turn  led  to  the  expansion  bulb  h.  This  part  of  the 
apparatus  had  a  capacity  of  approximately  100  cc.  and  served  as  a  pre- 
caution against  too  hasty  introduction  of  the  gas. 

The  adsorption  container  C  was  connected  to  the  expansion  bulb  by 
glass  tubing  and  a  ground  glass  joint  protected  by  a  mercury  seal.  The 
volume  of  this  bulb  together  with  that  part  of  the  connecting  tube  above 
the  mark  was  obtained  by  introducing  a  known  volume  of  dry  air  and 
measuring  the  pressure  developed.  Measurements  with  different  volumes 
showed  close  agreement  and  a  mean  of  these  values  was  used  for  calcu- 
lation purposes. 

The  manometer  needs  no  special  mention  except  that  it  was  found  de- 
sirable to  have  its  bore  identical  with  the  bore  at  I.  In  the  apparatus  first 
used  this  was  not  the  case  and  a  constant  correction  for  capillary  depression 
was  necessitated.  Pressure  readings  were  also  made  with  the  cathetom- 


eter  and  hence  all  such  readings  are  accurate  to  within  0.03  mm.  The 
mercury  well  controlling  the  manometer  was  worked  by  a  sensitive  ratchet. 
In  order  to  study  the  curve  formed  while  the  pores  were  being  emptied 
the  bulb  m  was  added  by  means  of  the  ground  glass  joint  o.  This  served 
as  a  holder  for  granulated  soda  lime  which  was  introduced  through  the 
mercury-sealed  ground  glass  joint  p.  The  stopcock  q  maintained  a  vacuum 
in  this  vessel  when  removed  from  the  apparatus  for  the  purpose  of  weigh- 
ing. The  electric  furnace  r,  previously  calibrated,  was  used  to  heat  the 
gel  to  the  required  temperature  during  evacuation. 

The  whole  apparatus  was  enclosed  in  a  completely  water-jacketed  air 
bath.  Three  gas  burners  under  the  bottom  furnished  rough  heating  ad- 
justment, while  a  system  of  8  carbon  lamps,  inserted  in  different  sections 
of  the  water  compartments  and  controlled  by  relays  and  a  sensitive 
toluene-mercury  regulator,  procured  very  close  temperature  control. 
This  bath  was  used  by  Morse  and  his  co-workers  in  their  measurement 
of  osmotic  pressure  at  high  temperatures  and  hence  is  described  elsewhere1 
in  the  literature.  Suffice  it  to  say  that  by  means  of  this  bath  the  tempera- 
ture was  maintained  constant  for  any  length  of  time  with  a  maximum 
fluctuation  of  less  than  0.05°. 

In  all  of  the  work  a  vacuum  was  maintained  by  using  in  series  a  rotary 
oil  pump  and  a  Gaede  high-vacuum  mercury  pump,  both  manufactured 
by  E.  Leybold.  A  MacLeod  gage,  K,  served  to  determine  when  evacua- 
tion was  complete,  such  being  considered  the  case  when  the  mercury 
threads  in  the  gage  became  level. 

Materials. 

All  the  mercury  used  in  this  investigation,  that  for  traps,  buret,  manom- 
eter and  gage,  was  thoroughly  cleaned  and  purified.  This  was  accom- 
plished by  first  allowing  it,  in  a  state  of  very  fine  subdivision,  to  fall  through 
2.4  meters  of  dil.  nitric  acid  for  5  or  6  times,  washing  with  distilled  water, 
then  caustic  soda,  and  finally  with  distilled  water.  After  drying  it  was 
redistilled  in  vacua. 

The  rubber  tubing  used  to  connect  the  mercury  wells  to  the  remaining 
part  of  the  apparatus  was  soaked  for  24  hours  in  dil.  sodium  hydroxide 
solution  in  order  to  remove  sulfur  present.  This  precaution  prevented 
premature  fouling  of  the  mercury. 

The  sulfur  dioxide  used  was  that  found  in  the  trade  and  was  taken  di- 
rectly from  its  metal  cylinder — a  method  recommended  by  Travers  in 
his  careful  work  on  purification  of  gases.  Of  course  its  purity  was  first 
tested.  This  was  done  by  immersing  a  100  cc.  inverted  buret  filled  with 
sodium  hydroxide  in  a  sodium  hydroxide  solution.  The  buret  was  now 
filled  with  sulfur  dioxide  from  the  cylinder,  and  after  a  short  time  was 
completely  absorbed  without  the  appearance  of  any  gas  bubble  at  the  top 
1  Am.  Chem.  J.,  48,  29  (1912). 


IO 

of  the  buret.  Several  experiments  were  also  made  from  a  sample  obtained 
from  the  same  cylinder  which  had  been  redistilled.  No  different  results 
were  observed.  A  further  check  on  the  purity  of  this  substance  was  ob- 
tained from  vapor-pressure  measurements.  No  change  in  pressure  being 
noticed,  no  matter  how  large  a  voume  of  gas  was  introduced.  Hence  the 
possibility  of  presence  of  oxygen,  nitrogen  and  carbon  dioxide,  the  most 
likely  impurities,  was  eliminated. 

All  of  the  gel  used  in  this  investigation  was  made  by  the  Davis,  Patrick 
and  McGavack1  process.  In  general  this  consists  in  allowing  an  acid  solu- 
tion and  a  solution  of  sodium  silicate,  both  solutions  being  kept  at  the 
proper  concentration,  to  mix  under  violent  agitation.  The  hydrosol 
"sets"  in  i  to  1 8  hours,  depending  upon  the  temperature  and  concentra- 
tion of  the  solution.  When  the  desired  state  of  firmness  is  reached  the 
material  was  washed  with  city  water,  the  washing  being  continued  until 
no  trace  of  electrolyte  could  be  detected  in  the  wash  water.  The  material 
was  then  dried  at  110°  in  vacuo  until  the  water  content  was  reduced  to 
7  or  8%.  By  this  method  a  large  amount  of  material  was  prepared. 

The  best  grade  of  sodium  silicate  solution  (water  glass)  furnished  by 
the  Philadelphia  Quartz  Company  was  used.  c.  p.  hydrochloric  was 
the  acid  used. 

In  order  to  remove  dust  particles  and  possible  metal  impurities  the  gel 
was  subjected  to  still  more  drastic  treatment.  This  was  accomplished 
by  saturating  it  with  nitric  acid  fumes  and  refluxing  with  c.  P.  cone,  nitric 
acid  for  12  hours.  The  material  was  then  washed  thoroughly  by  de- 
cantation  from  distilled  water  over  a  period  of  4  days.  This  part  of  the 
operation  cannot  be  hurried  or  accelerated  by  increasing  the  amount  of 
water  as  the  rate  of  diffusion  from  the  pores  of  the  gel  is  very  slow.  The 
material  was  then  dried  in  an  air  bath  at  110°. 

As  even  at  110°  a  large  amount  of  water  (16-24%)  still  remained  in 
the  gel,  and  as  uniform  samples  of  different  water  content  were  desired, 
some  arbitrary  process  had  to  be  employed  to  standardize  the  water  con- 
tent. This  was  accomplished  by  heating  a  mass  of  gel  for  different  periods 
of  time  under  a  vacuum  at  different  temperatures  For  instance,  Sample 
c  was  prepared  by  heating  for  one  hour  at  100-120°  and  for  3  hours  at 
300°.  Sample  d  was  heated  for  one  hour  at  100-120°,  one  hour  at  300°, 
and  finally  2  hours  at  500° — a  vacuum  of  i  to  5  mm.  being  maintained 
in  each  case  during  the  whole  time.  This  treatment  was  rigidly  held  to 
in  the  preparation  of  all  samples.  The  samples  were  then  put  in  glass- 
stoppered  bottles  and  these  in  a  sulfuric  acid  desiccator. 

All  water  determinations  were  made  by  heating  the  gel  in  a  platinum 
crucible  with  a  blast  lamp.  This  method  was  applicable,  as  water  was 

1  Reports  submitted  to  the  Chemical  Warfare  Service,  a  resume  of  which  will  be 
published  in  the  near  future. 


II 

the  only  volatile  component.  The  usual  method  for  obtaining  the  den- 
sity of  an  insoluble  (in  water)  solid  was  employed,  especial  care  being  used 
to  see  that  all  adsorbed  air  bubbles  were  removed.  Table  I  gives  the  ex- 
perimental results. 

TABLE  I. — WATER  CONTENT  AND  DENSITY  OF  DIFFERENT  SAMPLES. 

Sample  c.  d.  g.  f. 


Water,  %. 

Density. 

Water,  %. 

Density. 

Water,  % 

.    Density. 

Water,  %. 

Density. 

4-79 

2  .  1693 

3-53 

2.244 

2.36 

2.25" 

7.92 

2.123" 

4.82 

2.1604 

3-49 

2.236 

2.26 

>  •  .  . 

8.03 

4.90 

.... 

.... 

.  .  . 

.... 

.... 

8.07 

Mean  : 

4.87 

2.1648 

3  51 

2.240 

2.31 

.  ,  .  . 

8.OI 

.  .  . 

0  Calculated  from  values  obtained  from  c  and  d. 

Isotherms  were  made  at  — 80°,  — 54°,  — 34-4°,  — 33  4°,  o°,  30°,  40°, 
57°,  80°  and  100°.  For  +30°  and  +40°  the  constant  temperature  both 
surrounding  the  apparatus  was  used.  Solid  carbon  dioxide  contained  in 
a  Dewar  bulb  served  for  — 80°.  Liquid  ammonia  also  contained  in  a 
Dewar  bulb  and  with  an  arrangement  for  variable  pressure  served  for  the 
other  low  temperatures.  The  freezing  and  boiling  points  of  water  were 
used  for  o°  and  100°,  respectively.  The  vapor  of  boiling  acetone  and  ben- 
zene gave  the  points  57°  and  80°.  In  no  case  was  the  adsorption  bulb 
allowed  to  dip  in  the  boiling  liquid  itself  but  was  completely  bathed  with 
its  vapor.  The  flask  containing  this  liquid  fitted  tightly  at  the  top  around 
the  adsorption  bulb  and  had  openings  for  a  thermometer  and  also  a  long 
glass  condenser  which  avoided  the  necessity  of  continually  adding  liquid. 
In  all  cases  the  remaining  part  of  the  apparatus  was  kept  at  a  constant 
temperature  by  means  of  the  constant  temperature  bath. 

The  actual  temperature  points  of  the  2  low  degree  experiments  were 
fixed  by  the  aid  of  the  vapor-pressure  measurements  made  on  sulfur  di- 
oxide by  Steele  and  Bagster.1  These  investigators  furnish  the  only  meas- 
urements of  this  constant  at  low  temperatures  ( — 73°  to  — 36°)  and  when 
the  logarithms  of  these  pressures  are  plotted  against  the  absolute  tem- 
perature a  fairly  straight  line  results.  In  the  other  low  temperature 
runs  (Expts.  XXVIII  and  XXIX)  a  xylene  thermometer,  cali- 
brated recently  (1919)  by  the  U.  S.  Bureau  of  Standards,  was  used. 
The  corrected  readings  on  this  thermometer  were  — 33-4°  for  Expt. 
XXVIII  and  —34.4°  for  Expt.  XXIX.  The  vapor  pressures  ob- 
served in  these  runs  correspond  to  temperatures  — 37.8°  and  — 38.8° 
with  reference  to  the  Steele  and  Bagster  results.  Regnault,2  Pictet2 
and  Sajot,2  however,  have  measured  the  vapor  pressure  of  sulfur  dioxide 
from  — 30  to  -j-ioo°.  Their  results  are  in  good  agreement  with  each 
other  and  it  is  interesting  to  note  that  the  logarithmic  curve  plotted 

1  Steele  and  Bagster,  J.  Chem.  Soc.,  [2]  97,  2613  (1910). 

2  Results  tabulated  in  Landolt-BSrastein  "Tabellen." 


12 


from  them  when  extended  fixes  the  temperatures  in  question  at  —  34° 
and  —  35°,  respectively,  values  which  seem  to  be  the  true  ones. 

Procedure. 

The  gel  was  weighed  directly  into  the  adsorption  bulb  which  was  then 
attached  to  the  apparatus.  The  furnace  was  put  in  position  and  heating 
and  evacuation  were  commenced  at  the  same  time.  The  temperature 
and  length  of  heating  were  governed  primarily  by  a  consideration  of  the 
water  content  of  the  gel.  A  temperature  higher  than  that  used  in  the 
preparation  of  the  gel  was  never 
employed  —  this  was  done  so  as 
not  to  change  the  amount  of 
water  present.  The  evacuation 
was  continued  until  the  Mac- 
Leod gage  indicated  no  pressure. 
The  adsorption  bulb  was  then 
allowed  to  come  to  the  tempera- 
ture desired  and  the  first  intro- 
duction of  gas  was  made. 
Amounts  of  gas  such  that  points 
might  be  obtained  at  2,  5,  10, 
20,  30,  50,  60  and  70  cm.  were 
introduced.  After  introduction, 
the  mercury  level  was  brought 
to  point  /  (see  Fig.  i),  and  by 
reading  this  height  and  also  that 
on  the  manometer  itself,  the 
point  where  equilibrium  was 


reached   could    be    ascertained 

easily.  The  difference  between  these  2  readings  gave  the  pressure  of  the 
system.  In  the  same  manner  another  quantity  of  gas  was  introduced 
and  its  equilibrium  pressure  measured.  This  was  continued  until  atmos- 
pheric pressure  was  reached. 

For  points  on  the  reverse  curves  the  following  method  was  used.  The 
bulb  m  was  partially  filled  with  soda  lime  granules,  Stopcock  q  opened 
and  the  whole  system  thoroughly  evacuated.  After  removing  and  weigh- 
ing, the  bulb  was  again  attached  and  the  system  thoroughly  evacuated. 
The  mercury  controlling  the  MacLeod  gage  was  now  raised  to  a  point 
sufficient  to  cut  off  its  large  bulb.  Then  lowering  the  mercury  in  the  ex- 
pansion chamber,  h,  the  stopcocks  g  and  q  were  opened  and  gas  was  given 
off  from  the  gel.  When  sufficient  had  escaped  the  cock  g  was  closed  and 
the  mercury  in  h  raised  to  /.  The  pressure  gage  showed  almost  instant 
adsorption  by  the  soda  lime,  but  to  avoid  any  error  q  was  left  open  for 
an  hour  in  order  not  to  miss  the  last  traces  of  the  gas.  It  was  then  closed 


°  and 
760 


and  the  bulb  removed  and  weighed.     The  same  process  was  repeated 
for  every  point  desired.     Of  course  pressure  readings  were  made  for  every 

point  determined. 

All  pressure  readings 
were  corrected  to  o 
all  gas  volumes  to 
mm.  and  o°.  The  vol- 
ume of  the  gas  above 
the  gel  was  calculated 
each  time  and  subtracted 
from  the  amount  intro- 
duced. Knowing  the 
volume  of  the  bulb  C  to 
the  mark  /,  also  the  tem- 
perature and  pressure, 
this  value  was  easily  cal- 
culated from  the  gas 
laws.  When  the  bulb 
and  the  remaining  part 
of  the  apparatus  were 
at  different  temperatures 
the  volume  and  tern- 


.— 

-O 

--O 

J^ 

X* 

^ 

^x>" 

^>- 

D 

Jf 

^^ 

I 

^ 

f 

/cr 

x° 

c 

^ 

* 

^ 

*3> 

^fl 

X 

r^*^ 

^ 

^ 

x^ 

X 

,O 

J 

x 

^ 

x^ 

J$ 

^ 

^ 

^cr 

x^ 

>0 

"0- 
0 

-,** 

X 

\ 

^x 

x^ 

X 

k 

^x 

? 

^ 

^ 

^ 

X 

«* 

I 

X 

X 

X 

/J 

y 

, 

^ 

X 

x 

^ 

, 

X 

, 

X 

X 

X 

^ 

X 

^x" 

? 

X 

X 

' 

X 

X 

\ 

™ 

x 

X 

^ 

X 

X 

X 

X 

s 

# 

X 

x 

x 

" 

^y 

X 

X 

t 

X 

x 

^ 

X. 

X 

,x 

"<r 

^/ 

^ 

x 

d 

/ 

/ 

X 

/ 

/ 

cy 

/ 

^>Q 

3. 

-°S 

•p 

perature  of  each  part  were  considered  in  the  calculation. 

Experimental. 

The  results  for  Sample  c  of  the  gel  are  given  below. 
show  these  facts  graphically. 


Figs.  2  and  3 


Expt.  XII. 


2.4256 

p. 
105.88 

229.93 
397.00 
544.20 
671.50 


V0. 

18.24 
33.94 
51.32 
64.57 
73.65 


Vi. 

2.18 

4-74 

8.19 

11.23 

13-85 


X. 

16.06 
29.20 

43-13 
53-34 
59-80 


100  .        a    1.125. 

i  /»  =  0.745. 

X/M.             log  P. 

log  X/M. 

X/M  calc. 

6.62 

.02478 

0.82086 

6.50 

12.04 

•36159 

I  .  08063 

11.62 

17.78 

•59879 

1.24993 

17-43 

21.99 

•73576 

1.34223 

22.08 

24.65 

.82705 

I.39I82 

25.83 

2.6000  g.  (c). 


47.00 
192.19 
224.73 

407 . 88 
575-32 
671.95 


12.66 

47.60 

53-30 

80.73 

101 .09 

111.56 


Fi. 

i  .00 
4.09 
4.78 
8.70 

12  .24 
14.29 


Expt.  XIX. 
80°. 

X/M. 
4.48 

16.73 
18.66 
27.70 
34-17 
37-41 


X. 

11.66 
43-57 
48.52 
72.03 
88.85 
97-27 


a  2.239. 

log  P. 

0.67210 
•28373 
.35166 
.61053 

•75991 
.82733 


1/11=0.662. 


log  X/M. 

0.65128 
22350 
27091 
44248 
53364 
57299 


1/n. 
0.448 
0.680 
0.681 
0.678 
0.672 
0.669 


2.6500  g 

(<?). 

Expt.  XX. 
57°. 

a     5-755 

i/w=o.533. 

p. 

V0. 

Vi. 

X.              X/M. 

log  P. 

log  X/M.         l/n. 

18.97 

17.92 

0.42 

17.50         6  .  60 

0.27807 

0.91954        ..... 

46.16 

34.76 

1.03 

33-73        12-73 

0.66427 

.10483       0.534 

81.70 

50.42 

1.82 

48.60        18.34 

0.91222 

.26340       0.552 

168.84 

72.83 

3-75 

69.08       26.07 

1.22747 

.41614       0.534 

290.14 

97.38 

6-45 

90-93       34-31 

1.46261 

•53542        0.530 

615.65 

151.75 

13.68 

138.06       52.10 

I-78933 

.71684       0.534 

713.50 

164.51 

15-86 

148.65       56.09 

1.85339 

.74889       0.533 

Expt.  XVIII. 

2.600  g. 

to. 

40°. 

a     9-755- 

i  /w  =  0.50647. 

9-44 

19.72 

0.22 

19-50         7-50 

"1-97497 

0.87506      0.4560 

31-37 

43-63 

0-72 

42.91       16.50 

0.49651 

.21748      0.4596 

64.77 

67.14 

1-49 

65.65       25.25 

0.81137 

.40226      0.5090 

IIO.OO 

90.05 

2-54 

87-51       33-66 

1.04139 

.52711      0.5165 

189.13 

112.80 

3-90 

108.90       41.88 

i  .22822 

.62201      0.5152 

299.78 

151.28 

6.88 

144.40       55.54 

i  .47680 

.74461      0.5114 

448  .  60 

184.95 

10.30 

174.65       67.17 

1.65186 

.82718      0.5072 

567.52 

208.22 

13.04 

195.18       75.07 

I-75398 

•87547     0.5053 

692.20 

229.51 

15-88 

213.63       82.16 

i  .  84023 

.91466      0.5028 

Expt.  XXIII. 

1.7600  g. 

to. 

30°. 

a     12.93. 

i/w=o.485. 

p. 

V0. 

Vi. 

X.            X/M. 

log  P. 

log  X/M.    X/M  calc. 

9.50 

21.09 

0.23 

20  .  86        I  I  .  85 

1.97772 

I.OI372 

40.56 

46.24 

0.98 

45.26       25.72 

0.60810 

1.41027           25.32 

82.32 

66.25 

1-99 

64.26       36.51 

0.91551 

1.56241           35.94 

141.46 

87.50 

3-42 

84.08       47-77 

1.15063 

1.67916          46.74 

241.77 

113.78 

5.84 

107.94       61.33 

1.38340 

1.78767         60.  61 

408  .  02 

146.29 

9.86 

136.43       77-52 

i.  61068 

1.88941         78.10 

592.10 

175.60 

14.29 

161.31       91-65 

i  .  77240 

1.96213         93-59 

702.40 

191.72 

16.97 

174-75       99-29 

i  .  84650 

1.99691       101.60 

Expt.  XV. 

2.1422  g 

•  to. 

0°. 

a    29.14. 

i  /«=  0.43207. 

P. 

V0. 

Vi. 

X.              X/M. 

log  P. 

log  X/M.           l/n. 

17.67 

80.58 

0.44 

80.14          37.41 

0.24724 

.57299        0.4387 

34-92 

109.40 

0.87 

108.53          50.66 

0.54307 

.70467        0.4422 

59-32 

137.74 

1.48 

136.26          63.61 

0.77320 

•80353        0.4384 

88.16 

161.65 

2.21 

159.44        *74-43 

0.94527 

.87175        0.4307 

129.25 

190.26 

3-23 

187.03         87.31 

i.  11143 

.94106        0.4287 

179.46 

218.28 

4-49 

213-79         99-80 

•25396 

.99913        0.4263 

225.21 

242.18 

5-64 

236.54       110.42 

•35259 

2.04305        0.4277 

317.51 

283.25 

7-95 

275.30       128.51 

-50175 

2.10893        0.4291 

408  .  48 

321.18 

10.23 

310.95       I45-I5 

.61117 

2.16182        0.4327 

522.41 

363.63 

13.08 

350.55       163.64 

.71801 

2.21389        0.4361 

652.13 

397.64 

16.33 

381.31        178.00 

-81433 

2.25042        0.4331 

1-5440 
P. 
8.40 
36.50 
61.54 
80.47 
106  .  i  i 
145-39 
187.02 
258.17 
341-23 
422.16 
569.86 
639.09 

2.  2224  g. 

V0.                    Vi. 
39.08                  O.22 

78.98             0.94 
98.15             1*59 
111.19             2.08 
125-35             2.74 
143-59             3.76 
160.58             4.84 
186.42             6.68 
214.29             8.83 
240.04           10.92 
280.79           14-75 
297.98            16.54 

(c).                    o°. 

Bxpt  XVI.                           o°. 

X.                   X/M.                log  P.               log  X/M. 
25.86              25.57            1.92428               .40773 
78.04              50.54            0.56229               .70364 
96.56              62.54            0.78916               .79616 
109.11               70.66            0.90563               .84917 
122.61               79.4I               .02576               .89988 
I39.83              90.56               .16254               .95694 
155.74           100.86             .27189           2.00372 
179.74           116.41              .41185           2.06599 
205.46           133-06             .53305           2.12405 
229.12           148.39             .62548           2.17138 
266.04           172.30             -75577           2.23629 
281.44           182.27              .80556           2.26071 

Expt.  XXIV. 
d     29.89.                   1/71=0.4279. 

P. 

V0. 

rt 

X. 

X/M. 

Log  P. 

log  X/M  . 

1/n. 

10.43 

70.19 

0.26 

69 

•93 

31-47 

0 

.01828 

I 

•49790 

I 

.2248 

31.83 

I  I  I  .  04 

0.80 

no 

.24 

49.60 

o 

.50285 

I 

.  69548 

0 

•4374 

67.47 

152.69 

1.70 

150 

-99 

67.94 

0 

.82911 

i 

.93213 

0 

•  4301 

105-35 

182.47 

2.66 

179 

.81 

80.91 

.02263 

I 

.90800 

0 

.4248 

I47.05 

210.  16 

3-71 

206 

•45 

92.89 

.  16747 

I 

.96797 

o 

.4218 

190.59 

234-94 

4.81 

230 

•13 

103-55 

.28010 

2 

.01515 

0 

•4215 

245-57 

265.34 

6.20 

259 

•14 

116.60 

.39017 

2 

.  06670 

0 

-4252 

310.58 

293-37 

7-85 

285 

-52 

128.47 

.49217 

2 

.  10880 

o 

•4244 

366.57 

318.20 

9-23 

308 

•97 

139-03 

•56415 

2 

.14311 

0 

.4268 

467  .  79 

360.28 

11.82 

348 

.46 

156.79 

.67005 

2 

.19532 

0 

.4310 

585-96 

401.09 

14.80 

386 

.29 

173-82 

.76787 

2 

.24010 

0 

•4324 

720.15 

429-74 

18.18 

411 

-56 

185.18 

•85742 

2 

.26759 

0 

.4264 

Expt. 

XXVIII. 

i.  43io  g. 

(c). 

33-4°. 

a    "t 

'6.32".                 i/»=o.347i 

a 

6.00 

88.07 

o.n 

87 

-96 

61.47 

I 

.77815 

I 

.71866 

0 

-4234 

20.40 

132.28 

0.31 

131 

•  91 

92.18 

o 

.30963 

I 

.96484 

0 

•2657 

41.07 

180.24 

0.76 

179 

-48 

125.42 

0 

.61352 

2 

.09844 

0 

•3355 

72.00 

229.43 

1.29 

228 

.14 

159-42 

o 

.85733 

2 

.20254 

o 

•3735 

IT3-38 

270.82 

2  .04 

268 

.78 

187.82 

I 

-05453 

2 

-27375 

0 

.3709 

242.00 

313.54 

4-35 

309 

-19 

216.06 

I 

.38382 

2 

-33457 

0 

.3266 

243-00 

364.94 

4-37 

360 

-57 

251.97 

I 

.38561 

2 

.40138 

.  .  .  . 

243.01 

455-54 

4-37 

451 

•17 

351-28 

I 

.38562 

2 

.49869 

0  These 

constants  were 

calculated  from  all  points;  hence 

the  slope  of 

the  curve 

is  slightly  less  than 

indicatec 

from 

the  majority  of 

the 

observations. 

Expt 

.XXIX. 

i.  6660  g. 

(c}.                            —  34-  4°  • 

a 

72 

33- 

i/w=o.3794- 

4.22 

88.89 

0.07 

88 

.82 

53-3i 

I 

•52531 

i 

.72681 

o 

•3536 

13-85 

136.00 

O.22 

135 

-78 

81.50 

0 

•  I4H5 

i 

.91116 

0 

•3665 

29.02 

180.70 

0.47 

1  80 

-23 

108.1  8 

0 

.46270 

2 

•03415 

0 

•3778 

50.07 

229.31 

0-79 

228 

-52 

137-17 

o 

.69958 

I 

.13726 

o 

•3973 

73-45 

272.07 

1.18 

270 

-89 

162  .60 

0 

.  86599 

2 

.21112 

0 

.4051 

116.62 

321.86 

1.88 

319 

-98 

192.06 

I 

06677 

2 

.28344 

0 

•3975 

205.59 

347-35 

3-32 

344 

•  03 

206.50 

I 

.31300 

2.31492 

0 

•3470 

232.66 

426.30 

3-75 

432 

•55 

260.00 

I 

.36672 

2 

-4H97 

i6 
Expt.  XXVI. 


1.2876  g. 

(c). 

—  54  - 

a     112.7".          i  /W  =  0.405". 

p. 

V0. 

Fi. 

X. 

X/M. 

log  P. 

log  X/M. 

0.40 

40.17 

0.00 

40.17 

31-19 

2  .  60206 

1.49402 

4.16 

103.07 

0.06 

103.01 

So.OO 

Y.6I9O9 

1.90309 

9.85 

142.41 

o.  16 

142.25 

110.47 

T.99344 

2.04324 

17.55 

184.65 

0.28 

184.37 

143.19 

0.24728 

2.I559I 

27.50 

220.53 

0.44 

220.09 

170.93 

o  .  43933 

2.23282 

44.70 

268.31 

0.63 

267.68 

207.89 

0.65031 

2.31783 

89.00 

3I3-50 

1.44 

312.06 

242.36 

0.94596 

2.38435 

88  os 

•JK2    ^Q 

i  4.4. 

•ZCQ     OS 

28l    4.A 

<-"-*  •  ^O 

88.35 

OO^  •  Oy 
396.39 

T"T" 

1-44 

O3^J  •  ;/O 

394  .  95 

•**  *  «  T-T- 
307  .  85 

0  Constant  obtained  by  neglecting  that    point  when  saturation  was  nearly  reached. 
For  this  reason  i/n  is  slightly  larger  than  is  the  case  in  Expts.  XXVIII  and  XXIX. 


Vm. 


Expt.  XXV. 

I  .  6892  g. 

(c). 

—80°.         a\ 

B. 

V0. 

X/M.        log  P. 

log  X/M. 

0.13 

84.97 

50.30       2.11394 

I.70I57 

0.58 

166.92 

98.82       2.76343 

I  .  99484 

1.65 

249.71 

147.82 

.21748 

2.16973 

3-05 

307.17 

181.84 

.48430 

2.25969 

4.60 

349  •  i  i 

206.67 

.66276 

2.31528 

8.30 

388.23 

229.83 

.91908 

2.36138 

8.85 

433-54 

256.65 

.  94694 

2.40934 

8.80 

470.90 

278.77 

•  94448 

2.44526 

8.85 

512.35 

303.31       1.94694 

2.48187 

0  FI  correction  negligible;  b  not  calculated  as  saturation  pressure  is  less  than  i 
cm.  and  hence  no  comparable  values  would  be  obtained. 

In  order  to  make  clear  how  each  calculation  was  obtained  from  the 
actual  results,  and  exact  reproduction  of  Expt.  XXIII,  a  typical  example 
of  all  runs,  is  given  below. 

The  table  is  almost  self-explanatory.  The  meaning  of  the  symbols 
being  as  follows: 

B   =  barometer  reading  in  mm. 

Buret  =  readings  of  the  gas  buret  in  mm',  obtained  from  the  cathetometer  settings. 

F2  =  buret  readings  transformed  into  cc.  by  aid  of  the  calibration  curve. 

Fa  =  F2  corrected  to  standard  conditions,  o°  and  760  mm. 

Fo   =  difference  between  the  F3  readings,  or,  the  total  volume  of  gas  introduced 
in  cc.  and  under  standard  conditions. 

FI  =  volume  of  gas  in  the  vapor  phase  above  the  gel.  in  cc.  and  under  standard 
conditions. 

X  =  Fo  —  FI  total  volume  of  gas  adsorbed  in  cc. 

M  =  weight  of  the  gel.  in  g. 

X/M  =  volume  (cc.)  adsorbed  per  g.  of  gel. 

Time — In  this  column  is  given  the  time  of  introduction  of  the  gas  and  also  when 
pressure  readings  were  made. 

Pi  —  Pt  «=  uncorrected  pressure  of  the  system  in  mm.  of  mercury. 

P  =  pressure  of  system  in  mm.  of  mercury  corrected  to  o°  and  for  capillary  de- 
pression. 


17 

D  =  density  of  the  gel. 

T  =  temperature  of  the  constant  temperature  bath. 

Ti  =  temperature  of  adsorption  bulb.     At  +30°  and  +40°  T  =  TI. 

T2  =  weighted  mean  of  adsorption  bulb  temperature  and  that  of  the  remaining 
apparatus.  In  runs  at  +30°  and  +40°  Tz  =  Ti  =  T. 

F4  =  volume  (cc.)  of  the  adsorption  bulb  to  point  1  (see  Fig.  i).  F8  and  V\  were 
obtained  by  use  of  the  following  equations: 


F2  X  B  X  273        .  T7         (F4  —  M/P)PX  273 

72  =       760  xr      and  Fl  =  760  x  r2      - 

Expt.  XXIII. 

Weight  of  bulb  and  gel  17.  6478  g.     SO2  30°  Cap.  Depression  =    7.0000  mm. 

Weight  of  bulb                  15.  8878  g.                                     D  =     2.1648 

Weight  of  gel  (grams)       1.7600                                        V  =2i.o8cc. 

B.         Buret.        Vt.          Vt.            V0.           Vi.            X.       X/M.  Time.          Pi.            P2.       Pt  —  Pt. 

766.10  182.30  97.02  88.12     21.09    0-23     20.86  11.85  12.15   .................. 

765.30  312.80  73.88  67.03  ......................  i.  oo  201.70  185.34     16.36 

............................................  2.30200.95  184.45     16.50 

............................................  3-15  200.65  184.10     16.55 

765-30312.8073.8867.03     25.15    0.98     45.2625.72  3.15   .................. 

765.00471.2046.1841.88     46.24  ................  4.00252.90205.15     47.75 

............................................  4-50  252.82  205.05     47-77 

765.00471.2046.1841.88       20.01       1.99       64.2636.51       5.00    .................. 

764.70595.3324.1221.87    66.25  ................  5.40277.85188.10    89.75 

............................................  8.00278.25  188.60    89.75 

764.70595.3324.1221.87     21.25    3.42     84.0847.77  8.10  .................. 

764.95  726.88  0.68  0.62  87.50  ................  8.30  339.90  190.70  149.20 

............................................  9-45  339-90  190.70  149.20 

766.00177.3097.9088.90     26.28    5.84107.9461.33  9.45   .................. 

766.00  342.00  68.96  62.62  113.78  ................  10.45  439.75  190.95  248.80 

............................................  11.30  439.70  189.90  249.80 

............................................  12.30  439.10  189.07  250.03 

............................................  i.  oo  438.95  188.92  250.03 

766.00  342.00  68.96  62.62  32.51  9.86  136.43  77.52  i.io  .................. 

765.00544.6533.2030.11146.29  ................  3.15622.50205.40417.10 

............................................  4.00622.20205.05417.15 

765.00544.6533.2030.11    29.3114.29161.3191.65  4.05  .................. 

763.85725.75     0.88     0.80175.60  ................  5.05785.72181.95603.77 

............................................  5.50  785.60  181.70  603.90 

............................................  8.15  785.60  183.50  602.10 

............................................  8.45  785.80  184.00  601.80 

763.85  180.20  97.38  88.18  16.12  16.97  174-75  99-29  9.00  .................. 

763.15  281.00  79.65  72.06  191.72  16.97  ...........  9-45  887.40  176.80  710.60 

............................................  10.30  887.10  174.05  713.05 

............................................  n.oo  887.10  174.05  713.05 

Water  Content  and  Adsorption. 

It  was  known  from  previous  work1  that,  generally  speaking,  the  ad- 
sorption of  any  gas  was  dependent  upon  the  water  content  of  the  gel  used. 
The  fact  that  the  gas  or  liquid  was  soluble  or  insoluble  in  water  seemed 
1  Chemical  Warfare  Service  paper,  loc.  cit. 


i8 


to  make  no  difference.  It  was  also  pointed  out  in  this  paper  that  a  gel 
containing  from  6  to  9%  water  seemed  to  be  the  most  active.  Such 
observations  made  it  desirable  to  make  measurements  with  gels  of  differ- 
ent water  content. 

As  our  method  of  treatment  was  static  and  its  accuracy  depended  to 
a  very  great  extent  upon  complete  removal  of  all  air  before  the  run  was 
started,  we  were  limited  to  gels  of  very  low  water  content,  as  lengthy 
evacuation  and  high  temperatures  were  necessary  to  obtain  air-free  ma- 
terial. Gels  with  2.31,  3.51,  4.86  and  7.97%  water  were  used.  The 
results  are  given  below  and  are  also  shown  graphically  in  Fig.  4.  The 
sample  containing  2.23%  water 
is  practically  the  lower  limit,  it 
being  impossible  to  prepare  a 
sample  containing  a  smaller 
amount  of  water  and  at  the  same 
time  preserve  the  structure  of 
the  gel.  This  fact  may  be  used 
as  an  argument  that  a  small 
amount  of  the  water  in  the  gel  of 
silicic  acid  is  not  mechanically 
held,  but  is  in  some  way  inti- 
mately connected,  chemically  per- 
haps, with  the  silica  network.  A 
gel  containing  about  8%  water 
was  the  upper  limit,  as  with  this 
amount  at  room  temperature  the 
gel  has  no  vapor  pressure  and 
hence  fairly  good  evacuation 

without  any  appreciable  loss  of  water  could  be  accomplished.  The  curves 
speak  for  themselves,  the  isotherms  with  9.97  and  4.85%  lie  practically 
on  the  same  line,  indicating  that  the  maximum  value  of  adsorption  would 
be  possessed  by  a  gel  containing  an  amount  of  water  lying  between  these 
2  values.  This  further  confirms  the  statement  made  in  the  paper  pre- 
viously mentioned. 

The  fact  that  sulfur  dioxide  is  very  soluble  in  water  suggests  the  idea 
of  solubility,  that  is,  increased  water  content  should  cause  increased  ad- 
sorption. This  idea,  although  plausible,  is  contradictory  to  some  of  the 
observations,  for  it  has  been  shown  that  there  is  a  maximum  water  con- 
tent above  which  adsorption  decreases  and  does  not  increase.  Further- 
more, even  in  those  cases  where  adsorption  does  increase  with  greater 
water  content,  the  increase  is  entirely  too  large  to  be  accounted  for  by 
solubility.  For  instance,  the  average  difference  in  X/M  for  Samples 
g  and  d  was  7  cc.  The  actual  difference  in  the  amount  of  water  was 


.» 

->** 

.••S 

0^ 

^ 

V* 

s 

d 

t 

^ 

/ 

sf 

3 

,/ 

/ 

s 

~s 

^, 

^ 

.-/ 

/I 

S 

*•<, 

'•/ 

/ 

^ 

8 

— 

n'/ 

/ 

nj 

/ 

^'V 

H 

// 

/ 

/ 

\L 

r;n 

ion 

•f 

m 

rpt 

on 

Wit 

,J 

/ 

/ 

' 

Vat: 

_r  Li 

j 

7 

/  , 

/ 

a 

_ 

J 

1U 

i  n 

jz 

2 

\ 

. 

. 

V 

? 

f 

. 

0| 

t 

'/ 

<r 

, 

HI 

•!// 

3  c 

•n 

14 

19 

O.OI2O  g.,  which  would  adsorb  at  40°  about  0.5  cc.  of  sulfur  dioxide,  a 
value  far  too  low  for  the  difference  actually  observed. 

It  is  believed  that  this  difference  in  adsorption  with  small  changes  in 
water  content  might  be  due  to  the  change  caused  in  the  size  of  the  pores. 
If  the  water  content  is  too  low  we  have  the  pores  too  large  and  hence  the 
capillary  forces  acting  are  enormously  diminished  and  cause  low  values 
for  adsorption.  On  the  other  hand,  if  the  water  content  is  too  high  we 
have  the  smaller  capillaries  partially  filled  and  hence  the  space  available 
for  the  gas  is  decreased.  An  adjustment  of  these  2  factors  must  be  made 
to  produce  the  best  results. 


Expt.  XVII. 

2.4118 

g.  (g)- 

2.3i%H20. 

40°. 

a  =  2 

[.936.            i/n 

=  0.678. 

p. 

Fo. 

V\. 

X. 

X/M. 

log  X/M  . 

log  P. 

14.   60 

IO   0^ 

O    34 

IO.  SO 

A      -5Q 

A  if.  .  \J\J 
51-35 

±\j  •  VO 

28.96 

*•  •  OT- 
I.I9 

^  *  \)7 

27.77 

f.  •  Oy 
II-5I 

.06108 

0.71054 

90.53 

44.28 

2.09 

42.19 

17-49 

.24279 

0.95679 

134-10 

59-04 

3-10 

55-94 

23.19 

•36530 

i  .  12743 

204.85 

79-35 

4-73 

74.62 

30.94 

.49052 

I.3H43 

304.42 

103.66 

7-03 

96.63 

40.06 

.60271 

.48347 

409.16 

125-37 

9-45 

115.92 

48.06 

.68178 

.61189 

552.77 

I53.I7 

12.77 

140.40 

58.21 

•76507 

.74255 

626.57 

165.65 

14-47 

151.18 

62.68 

.79713 

.  79696 

701.50 

177.29 

16.21 

161.08 

66.79 

.82471 

.84603 

Expt.  X. 

I.892I 

g-  (<*). 

3.51  %H2O. 

40°. 

a  = 

5.821.           i/n 

=  0.600. 

P. 

V0. 

Ki. 

X. 

X/M. 

log  P. 

log  X/M  . 

45.76 

27.68 

i  .06 

26.62 

14.07 

•  65049 

I  .  14829 

137-34 

57-73 

3-20 

54-53 

28.82 

.  13780 

.45969 

263.77 

87.62 

6.14 

81.48 

43-06 

.  42  i  i  i 

•  63407 

400.32 

112.75 

9-33 

103.42 

54-66 

.60241 

.73767 

549-71 

134-41 

12.  8l 

121.60 

64-37 

•74014 

.80868 

644.89 

150.05 

15-03 

135-02 

71.36 

.  80946 

.85345 

Expt.  XIV. 

2.9980     g. 

(/)•            8 

01%  H2O. 

40°. 

a  = 

8.129.          i/n 

=  0.555. 

P. 

V0. 

Vt 

X. 

X/M. 

log  X/M. 

log  P. 

8.68 

17.49 

o.  19 

17.23 

5-74 

25-25 

37-02 

0-57 

36.45 

*/  "  *  *r 
12  .  l6 

•08493 

0.40226 

65.55 

64.00 

i-49 

62.51 

20.85 

.31911 

0.81657 

88.20 

85.61 

2.00 

83.61 

27-89 

•44545 

0-94547 

143-57 

114.06 

3-26 

110.80 

36.95 

.56761 

.  15706 

232.02 

154.21 

5-27 

148.94 

49-68 

.69618 

.36551 

294.44 

170.36 

6.69 

163.47 

54-52 

•73656 

.46907 

388.54 

196.24 

8.83 

187.41 

62.51 

•79595 

•58943 

533-13 

230.77 

12  .  II 

218.66 

72.93 

.86291 

.72683 

651.00 

254-91 

14-79 

240.12 

80.09 

1.90358 

.81358 

It  was  noticed  that  when  the  same  charge  was  used  for  another  run 
the  amount  adsorbed  was  distinctly  less  than  in  the  original  run.     This 


20 


was  due,  without  doubt,  to  the  fact  that  it  required  more  drastic  treat- 
ment, longer  evacuation  and  higher  temperature,  to  remove  the  sulfur 
dioxide  than  it  did  in  the  case  of  the  air  originally  present.  During  this 
process  a  small  amount  of  water  was  removed  and  the  result  followed 
along  the  lines  we  have  just  discussed,  decreased  adsorption.  In  agree- 
ment with  this  conclusion  is  the  further  fact  that  where  the  gel  originally 
started  with  was  of  low  water  content  there  was  less  difference  between 
the  first  and  second  run.  The  following  examples  will  show  this  more 
clearly.  Compare  Expt.  XI  with  X,  and  Expt.  IX  with  XVIII. 

Expt.  XI. 


1.8921  g/ 

p. 

32.43 
89-93 
166.08 
265.11 
407.48 
572.81 
672.04 


v0. 
22.28 

43-44 

64.58 

85.21 

110.13 

134-95 
147.22 


30". 
Ft. 

0.75 
2.08 

4-31 

6.20 

9.48 
13.35 
15.66 


X. 

21.53 
41.36 

60.27 

79-01 

100.65 

131.60 

131-56 


0  Previously  used — originally   (d)   water  content. 

Expt.  IX. 


2.1985  g.a. 


8.727. 


X/M. 
H-37 
21.85 
31-85 
41-75 
53-19 
64.27 

69-53 


i/n  =  0.5260. 


p. 

V0. 

Vi. 

X. 

X/M. 

log  P. 

log  X/M. 

1/n. 

20.90 

27.64 

0.50 

27.14 

12.34 

0.32015 

1.09132 

0.4699 

74.20 

57-44 

1.72 

55-72 

25.38 

0.86451 

1.40449 

0.5362 

152.00 

85.68 

3-55 

82.13 

37-31 

1.18184 

1-51183 

0.5169 

335.48 

130.87 

7-79 

123.08 

55-98 

1.52566 

I  .  74803 

0.5290 

574-74 

173-34 

13-33 

160.01 

72.78 

1-75947 

1.86201 

0.5232 

0  Previously  used — originally  (c)  water  content. 

Adsorption  Reversible. 

All  earlier  work  on  the  adsorption  of  vapors  by  silicic  acid  gel  showed 
a  marked  difference  in  the  amount  adsorbed  at  the  same  pressure  and 
temperature,  depending  upon  whether  the  pores  were  being  filled  or 
emptied.  The  earlier  work  of  van  Bemmelen1  with  water  and  later  that 
of  Anderson2  with  water,  alcohol  and  benzene,  all  showed  this  wide  differ- 
ence in  the  filling  and  emptying  process.  They  explained  this  hysteresis 
from  the  known  fact  that  a  liquid  in  a  capillary  tube  has  a  greater  vapor 
pressure  when  being  filled  than  when  being  emptied,  as  in  the  former 
case  we  have  a  diminution  of  the  curvature  of  the  liquid  meniscus  due  to 
incomplete  wetting.  This  is  a  very  plausible  explanation  as  well  as  an 
interesting  example  of  capillary  phenomena.  So  it  was  thought  de- 
sirable to  obtain  isotherms  where  the  sulfur  dioxide  was  removed  from 
the  gel  instead  of  being  added. 

As  has  been  stated  above,  this  was  accomplished  by  opening  a  carefully 

1  £.  anorg.  Cham.,  13,  233  (1897);  18,  98  (1898). 

2  Loc.  cit. 


21 


evacuated  soda-lime  bulb  to  the  system  and  when  sufficient  had  been 
taken  up  removing  the  same  and  weighing.  The  weight  was  then  changed 
to  cc.  at  standard  conditions  by  using  the  proper  conversion  factor.1  All 
reversible  measurements  were  made  at  o°,  as  here  we  have  a  very  large 
adsorption  and  the  difference,  if  any,  would  for  that  reason  be  magnified. 
Expts.  XXXII  and  XXXIII  were  the  first  reversible  runs  made. 

Expt.  XXXII. 


1.  1  140  g.  (c). 


a  =  32.95. 


i/n  =  0.4116. 


Filling  Pores. 


P.                     F0.                  Fi.                  X.              X/M. 

log  P. 

log  X/M.             l/n. 

19.05           44-27          o. 

54           43-73         39-25 

0.27989 

1.59074        0.2716 

52.10              78.16             I. 

18           76.98         69.10 

0.71684 

1.83048        0.4487 

102.40            100.54             2- 

32           98.22         88.17 

.01030 

1-94532         0.4231 

188.40           127.13             4. 

28         122.85       110.28 

.27508 

2.04250        0.4115 

291.00           153.32            6. 

59         146.73       131-71 

.46389 

2.11962         0.4111 

453-50          189.12         10. 

27         178.85       160.10 

.65658 

2.20439        0.4144 

704.80          225.25         15. 

96         209.29       187.87 

.  84807 

2.27387        0.4091 

Emptying  Pores 

a  =  37.13. 

i/n  =  0.38107. 

P.                Wt.  SO2. 

Vol.  S02.              F0. 

Vi. 

X.                 X/M. 

704  .80          o  .  oooo 

o.oo          225.25 

15-96 

209.29           187.87 

445.76          0.1060 

37.05          188.20 

10.12 

178.08           159.86 

286.70          0.2060 

72.00         153.25 

6.50 

146.75           131.73 

149.96          0.3095 

108.18           117.07 

3-50 

113.57           101.95 

77.49          0.3828 

133-79            91-46 

2.21 

89.25               80.  12 

19.00          0.4870 

170.21             55-04 

0.44 

54.60               49.01 

4.25          0.5459 

190.80            34.45 

0.10 

34-35           30.85 

P. 

log  P. 

X/M. 

l/n. 

704  .  80 

i  .  84807 

2.27387 

0.3810 

445  •  76 

i  .64910 

2.20374 

0.3844 

286.70 

1-45743 

2.11969 

0.3766 

149.96 

I.I7598 

2.00838 

0.3738 

77-49 

0.88925 

1.90374 

0-3755 

19.00 

0.27875 

1.69028 

0.4324 

4-25 

1.62939 

1-53593 



Expt.   XXXIII 

2.6005  g-  (c). 

o°.                  a  =  29 

222. 

i/n  =  0.4231. 

Filling  Pores. 

P.                    F0.               Fi.                 X.              X/M. 

log  P. 

log  X/M.            l/n. 

12  .1               84.  17            O 

•27^      83.90        31.67 

0.08422 

1.50065        0.4166 

63.87          I7I.II             I 

•39       169.72         65.26 

o  .  80530 

1.81465        0.4334 

115.00         2l6.II             2 

•55       213.56         82.12 

.  06070 

1.91445        0.4232 

176.40       258.33         3 

•75       254.58         97.90 

.24650 

1.99078        0.4213 

265.07       307-42         5 

•77       301.65        116.00 

.42336 

2  .  06446         0  .  4207 

349.26       349.64         7 

.61       342.03       131.52 

.54315 

2.II899         0.4234 

453-00       396.16         9 

.87       386.29       148.54 

.65610 

2.17185         0.4265 

561.44       437-18       12 

.23       424-95        163-42 

.74930 

2.21330         0.4268 

745-95       474-81        16 

•23       458.58       176.35 

.87792 

2.24637         0.4157 

1  Landolt-Bornstein,  ' 

'Tabellen,"  one    liter  of 

sulfur    dioxide    at  sea-level,    760 

22 


a  =  30.605                            Emptying  Pores. 

P. 

Wt.  SO.              Vol.  SO2. 

V0. 

745-95 

O.OO 

474.81 

545  •  70 

0.1062              33-12 

437.69 

338.91 

0.3614            126.32 

348.49 

163.55 

0.6386            223.20 

25I.6I 

50.17 

0.9106           318.27 

156.54 

P. 

log  P. 

log  X/M. 

745.95 

1.87792 

2.24637 

545  •  70 

I  •  73695 

2.21418 

338.91 

1.53008 

2.21783 

163.55 

1.21365 

1.97950 

50.17 

o  .  70044 

1.77656 

i/n  =  0.4119. 


Vi.         X. 

X/M. 

16.23   458.58 

176.35 

11.85   425.84 

163.75 

7.38   341.11 

131.17 

3.56   248.05 

.95-39 

1.09   155.45 

59-78 

I/a. 

(X/M)p. 

o  .  4050 

.... 

0.4191 

161.10 

0.4130 

130.00 

o  .  4068 

94.40 

0.3985 

67.50 

In  the  last  column,  marked  (X/M)F,  is  given  the  amount  adsorbed  on 
filling  for  the  same  pressure  values  observed  for  the  emptying  process. 
In  every  case  the  former  is  the  smaller  value.  Although  this  difference 
is  small,  nevertheless  it  is  real.  Doubt  was  at  once  raised  whether  all 
the  air  could  be  removed  by 
the  treatment  used.  If  not, 
the  first  introduction  of  sulfur 
dioxide  would  liberate  the  air 
present  and  thus  cause  an  in- 
creased pressure.  On  the  first 
exposure  to  the  soda-lime  bulb 
practically  all  of  the  air  would 
rush  out  and  hence  the  pres- 
sure due  to  the  air  on  the 
ascending  curve  would  be 
eliminated  and  a  greater  ad- 
sorption at  the  same  pressure 
would  be  observed.  As  a 
matter  of  fact,  after  the  first 
exposure  the  MacLeod  gage, 
used  to  indicate  when  all  gas 
had  been  adsorbed,  never  showed  a  vacuum,  but  indicated  the  presence 
of  0.1-0.2  cc.  of  gas.  This  was  not  noticeable,  or  if  so,  very  slightly 
after  the  first  exposure. 

It  was  now  decided  to  prepare  an  absolutely  air-free  sample  even  at 
the  expense  of  making  a  gel  of  only  approximately  known  water  content. 
This  was  accomplished  by  allowing  the  gel  to  stand  in  equilibrium  with 
sulfur  dioxide  at  about  70  cm.  pressure  overnight  and  then  pumping  it 
off  and  repeating  the  process.  This  was  done  4  times  and  it  is  safe  to  say 
that  the  gel  was  completely  freed  from  air.  Two  experiments  were  run 
with  a  sample  thus  prepared.  The  results  are  given  on  p.  964  and  are  shown 
graphically  in  Fig.  5. 


2-3200  g.a. 

Expt.    XXXIV 
o°.                      a  =  21 
Filling  Pores. 

-943- 

i  In  =  0.4910. 

P. 

V 

Vi.                   X. 

X/M. 

log  P. 

log  X/M. 

l/«. 

33-37 

93 

.12               0 

73           92.39 

39-82 

0.52336 

1.60010 

o  .  4944 

131.14 

182 

.60               2 

87         179-73 

79.27 

•II773 

1.89911 

0.4990 

215-79 

232 

.01           4 

72         227.29 

97-93 

-33403 

1.99092 

o  .  4869 

305-14 

276 

46           6 

68         269.78 

116.28 

.48450 

2.06551 

0.4878 

413-20 

324 

66           9 

04         315-62 

136.04 

.61616 

2.13366 

o  .  4902 

529.44 

369 

68         ii 

58         358.10 

154-35 

.72382 

2.18851 

0.4914 

722.77 

412 

46         15 

82         396  .  64 

170.96 

.85900 

2.23290 

0.4790 

Emptying  Pores. 

a  =  21.943. 

i/n  =  0.4910. 

p. 

Wt.  SO2. 

Vol.  SO2. 

V.. 

Vi. 

X. 

X/M. 

722.77 

.... 

.... 

412.46 

15.82 

396.64 

170.96 

498.27 

0.1554 

54-31 

358.15 

10.90 

347-15 

149.68 

341-10 

0.3399 

118.80 

293.66 

7-47 

286.19 

123.36 

209.71 

0.5219 

182.41 

230.05 

4.70 

225-35 

97-13 

85-25 

0-7559 

264.20 

148.26 

1.86 

146.40 

63.10 

p. 

log  P. 

log  X/M. 

1/n. 

722. 

77 

1.85900 

2.23290 

0 

•4790 

498 

27 

1.69747 

2.I75I7 

0 

-49I3 

341- 

10 

1.53288 

2.09118 

0 

•4957 

209. 

7i 

1.32162 

1.98735 

o 

.4888 

95- 

25 

0.93069 

i  .  80003 

0 

.4928 

Sample   (c)  repeatedly  evacuated. 


2.3200  g. 


Expt.  XXXV. 

a  =  21.49. 
Filling  Pores. 


i/n  =  0.4966. 


P. 

V0.            \ 

X. 

X/M. 

log  P. 

log  X/M. 

1/n. 

X/M 

calc. 

31 

61 

go.  i  i      o 

67 

89.44 

38.55      0.49982 

I 

.58602 

0.5078 

38. 

06 

130 

54 

183.00      2.94 

180.06 

76.07 

•II574 

I 

.88121 

0.4913 

75. 

21 

205 

28 

227.85       4 

35 

223.50 

96.34 

.31235 

I 

.98381 

o  .  4966 

96. 

37 

296.21 

274-51       6 

28 

268.23 

115.62 

.47160 

2 

.06303 

0.4965 

US- 

62 

397 

70 

320.47       8 

43 

312.04 

134.51 

.59956 

2 

.12879 

0-4977 

133. 

83 

508 

66 

363.98     10 

73 

353-20 

152-24 

.70643 

2 

.18253 

0.4983 

151. 

23 

648.57 

405.53     H 

19 

391-34 

168.67 

.81196 

2 

.22704 

0.4944 

170. 

62 

Emptying  Pores. 

a  = 

21.943. 

i/n  —  0.4910, 

P. 

Wt.  S02. 

Vol.  S02. 

V. 

V. 

X. 

X/M. 

648 

S7 

i 

_    _  . 

19 

•7QI 

168. 

6? 

V^T.^ 

445 

o  / 
34 

0.1878 

65-64 

339.89 

9 

•   *  7 

•43 

OV  A 

330 

•46 

142. 

v  / 

44 

323 

oo 

0.3381 

118.17 

287.36 

6 

.63 

280 

•73 

121  . 

00 

201 

84 

0.5128 

179.23 

226.30 

4 

.27 

222 

•03 

95. 

70 

96 

46 

0.7118 

248.79 

157.74 

2 

.04 

155 

.70 

67. 

ii 

30 

89 

0.9058 

316.59 

88.94 

O 

•65 

88 

.29 

38. 

06 

P. 

648.57 

445-34 

323.00 

201.84 

96.46 

30.89 


log  P. 
1.81196 
1.64869 
1.50920 
1.30501 
0.98435 
0.48982 


1/rz. 

0.4888 
0.4926 
0.4912 
0.4901 
0.4981 
0.4881 


(X/M)p. 


I43.OO 

121.20 

95  .80 

66.60 

37.8o 


log  X/M. 
2.22704 
2.15363 
2.08279 
1.98091 
1.82679 
1.58047 
0  Charge  of  Experiment  XXXIV  evacuated. 

The  agreement  is  well  within  the  limit  of  experimental  error.  In 
other  words,  the  adsorption  of  sulfur  dioxide  by  silicic  acid  gel  is  a  re- 
versible process. 

It  will  also  be  noticed  that  the  absorption  values  do  not  agree  with 
those  previously  made  at  this  temperature.  The  reason  for  this  may  be 
found  in  the  discussion  given  under  the  head  of  water  content  and  ad- 
sorption. Here  it  was  shown  that  by  repeated  exhaustion  of  the  gel  the 
water  content  of  the  gel  is  decreased  and  hence  its  adsorptive  power  (in 
this  case)  is  at  the  same  time  lowered. 

In  order  to  prove  more  conclusively  that  minute  traces  of  air  were 
responsible  for  the  lack  of  reversibility  a  sample  was  run  where  there 
was  a  definite  amount  of  air  present.  This  was  done  by  evacuating  the 
bulb  but  a  short  space  of  time.  To  be  exact,  there  was  at  the  beginning 
of  the  run  a  partial  pressure  of  air  of  0.7  mm.  The  experiment  was 
carried  out  in  exactly  the  same 
manner  as  previous  reversible 
runs.  The  experimental  facts 
are  given  in  Table  II  and 
Expt.  XXXVII.  They  are 
also  shown  graphically  in  Fig.  ISO 
6.  Table  II  is  given  to  show 
the  large  effect  of  small 
amounts  of  air  upon  the  rate 
of  adsorption.  With  air  pres- 
ent, as  may  be  seen,  it  is  a 
question  of  hours  before  equi- 
librium is  reached,  while  under 
conditions  of  a  perfect  vacuum 
equilibrium  is  reached  in  a  very 
few  minutes.  The  fact  that 
there  is  an  appreciable  time 
factor  at  all  in  the  latter  case  is  caused  chiefly  by  the  time  necessary  for  the 
dissipation  of  the  heat  evolved  during  adsorption.  In  Fig.  7  are  plotted 
the  rates  of  adsorption  in  the  form  of  dp/dt  for  two  points,  one  obtained 
in  the  presence  of  air  and  the  other  in  the  absence  of  air.  This  gives 
another  strong  evidence  of  what  important  role  air  plays  in  adsorption 


te 


25 

phenomena.  The  presence  of  this  substance  is  suggested  as  a  possible 
reason  for  the  hysteresis  observed  by  previous  workers  on  reversible  ad- 
sorption isotherms. 


180 


Expt.  XXXVII.0 


2.7430  g.  (c). 


i/n  =  0.4569. 


Filling  Pores. 

p. 

V9. 

Ft. 

X. 

X/M. 

log  P. 

log  X/M  . 

25.28 

56.97 

o.54 

56.43 

20.57 

0.40278 

1.31323 

46.05 

104.32 

0.99 

103.33 

37.60 

0.66323 

I.575I9 

69.20 

148.09 

1.49 

146  .  60 

53-44 

0.84011 

1.72787 

148.37 

230.27 

3.19 

227.08 

82.78 

1.17135 

1.91793 

284.86 

320.64 

6.13 

314-51 

114.66 

1.45463 

2.05941 

424-55 

396.48 

9.12 

387.36 

141.22 

1.62793 

2  .  14989 

636.35 

487.38 

13.67 

473.71 

172.70 

I  .  80369 

2.23729 

Emptying  Pores. 

i/n  =  0.4012. 

P. 

Wt.  SO2. 

Vol.  SO2. 

Fo. 

Vi. 

X. 

X/M. 

636.35 

0.0000 

0.00 

487.38 

13.67 

473-71 

172.70 

425-50 

0.2114 

73.89 

413.49 

9.15 

404.34 

147.41 

318.13 

0.3711 

129.71 

357.67 

6.68 

350.99 

127.96 

181.71 

0.5999 

209.67 

277.71 

3-90 

273-81 

99-82 

107.64 

o.757i 

264.62 

223.76 

2.31 

221.45 

80.73 

27.66 

0.9818 

343.15 

144-23 

0.58 

i43  •  65 

52-37 

P. 

log  P. 

log  X/M. 

(X/M)p. 

636.35 

.  80369 

2.23729 

.... 

425-50 

.62890 

2.16853 

I4L50 

318.13 

.50264 

2  .  IO7O7 

122  .OO 

181.71 

.25937 

1.99922 

92.50 

107.64 

.02197 

I  -  90703 

72.00 

27.66 

0.44185 

I  .71908 

25-00 

0  Partial 

pressure  of 

air  at  beginning  of  experiment  of  0.7 

mm.  of 

Hg. 

26 


TABLE  II. 

XI  M                       Time  (min.). 

f  40 

2O.  S7...                    .1  \A-*>. 

TV/ 

185 

215 

40 

70 

77   6O                                            .   ' 

IOO 

130 

205 

15 

45 

75 

5  ^  4.4.  .  .                     »  .  ' 

JQC 

135 

165 

195 

15 

30 

60 

82  78  ...         .  .  < 

oo 

y\_/ 

125 

150 

1  80 

f  is 

1  45 

114.  66  ..                 .  S  75 

250 

[280 

15 

30 

141  22  .  .  .  < 

125 

155 

185 

215 

85 

172  7O 

150 
215 

255 

Total  time.  .            25  hours.  4.5  minutes. 

Pressure  (mm.). 
30.60 
26.40 
25-80 
25-30 
25-30 

51.50 
47-70 
46.75 
46.25 
46.20 

93-90 
75-95 
71.00 
69.80 
69.65 
69.40 
69.45 

195-99 
170.50 
153-10 
149.35 
148.70 
148.85 
148.85 

343-65 

308 . 20 
291.05 
285.75 
285 . 80 

500.10 
461.40 
428.40 
425.00 
426.05 
425.95 

656 . 6O 
640.17 
638.50 
638.45 


Discussion. 

Certainly  there  must  be  a  mathematical  interpretation  possible  and 
from  the  well  defined  regularity  and  similarity  of  the  curves  this  appears 
to  be  far  from  complicated.  A  brief  review  of  those  equations  in  general 
use  is  certainly  appropriate. 


27 

Many  adsorption  formulas  have  been  proposed.  That  of  Arrhenius,1 
later  amplified  by  Schmidt,2  is  certainly  logical  and  has  been  used  over  a 
wide  range  of  cases.  It  has  the  following  form  when  applied  to  gases: 


where  p  is  the  pressure  of  the  gas,  5  the  amount  adsorbed  at  saturation 
per  gram  of  substance,  x  the  amount  adsorbed  at  the  different  pressure 
intervals,  K  and  A  are  constants  and  e  has  its  usual  value.  Changing 
this  somewhat,  we  may  write 

PS 


which  states  that  the  amount  adsorbed  is  equal  to  the  product  of  the 
pressure,  the  saturation  value  and  a  constant,  itself  a  function  of  the  tem- 

perature, which  fact  is  expressed  by  the  power  —  -^  —  -  -  to  which  e  is 

raised.     Written  in  the  logarithmic  form, 

^4  (5  _  %\ 
log  p  —  log  5  =  log  K  —  log  x  —  -^—  •  g  —  -  log  e, 

since  log  e,  A  and  5  are  constants,  and,  as  Schmidt  has  shown,  log  K  =  k  — 
log  S,  the  expression  is  simplified,  giving 

log  p  —  log  x  —  B(S  —  x)=k. 

This  gives  an  equation  well  suited  for  calculation  purposes.  The  re- 
sults of  adsorption  of  sulfur  dioxide  by  silica  gel  fits  excellently  this  equa- 
tion when  the  isotherms  at  the  higher  temperatures  are  used,  those  above 
o°.  Even  those  at  the  lower  temperatures  give  fairly  satisfactory  results 
if  proper  manipulation  of  the  constant  B  is  made.  The  value  of  k  in- 
creases with  the  temperature  while  there  is  a  tendency  for  B  to  remain 
constant,  although  this  also  seems  to  increase  with  temperature.  Theo- 
retically B  should  remain  unchanged  throughout  the  temperature  range. 

A  great  drawback  to  this  equation,  as  has  been  pointed  out  before  by 
Marc,3  is  that  it  is  too  pliable.  For  instance,  fixing  arbitrarily  the  value 
of  5  the  constant  B  may  vary  through  wide  limits  and  still  fit  the  observa- 
tions. Also,  the  value  5  can  be  changed  at  will  and  by  slight  changes  in 
B  and  k  the  observations  are  again  correlated.  Another  objection  is  the 
fact  that  5  is  not  a  constant  through  a  wide  temperature  range.  It  is 
logical  to  believe  that  it  must  vary  with  the  density  of  the  condensed  gas. 
This  correction  would  be  considerable  and  would  give  another  variable 
to  contend  with  in  the  Schmidt*  equation. 

The  adsorption  ideas  of  Langmuir4  in  their  present  form  are  not  applica- 

1  S.  Arrhenius,  Medd.  K,  Vetenskapsakad.  Nobelinst.,  2,  7  (1911). 

2  G.  C.  Schmidt,  Z.  Phys.  Chem.,  78,  667  (1912). 
8  Marc,  ibid.,  81,  679  (1913). 

4  J.  Am.  Chem.  Soc.,  39,  1848  (1917);  40,  1361  (1918). 


28 

ble  to  the  measurements  of  adsorption  by  porous  bodies.  The  stray  field 
of  force,  eminating  from  the  surface  of  the  adsorbent,  it  is  believed,  reaches 
out,  attracts  and  holds  those  molecules  of  the  gas  that  approach  its  sur- 
face. The  maximum  adsorption  is  reached  when  this  surface  is  covered 
by  a  film  of  the  adsorbed  substance  which  is  but  a  molecule  in  thickness. 
Hence,  from  this  theory,  other  factors  being  equal,  adsorption  is  dependent 
primarily  upon  the  amount  of  surface  exposed.  The  fact  that  the  pressure 
of  the  gas  phase  changes  gradually  is  ascribed  to  the  difference  in  the 
strength  of  the  individual  lines  of  force  given  off  from  the  surface.  Much 
evidence  is  brought  forth  to  support  this  conception.  Thus,  in  order  to 
apply  the  formula  to  porous  bodies  a  measure  of  the  internal  surface 
would  be  necessary.  The  difficulty  of  such  an  undertaking  is  easily 
seen.  It  is  true  that  a  rough  approximation  might  be  arrived  at  by 
making  ultramicroscopic  measurements  of  the  size  of  the  pores,  such  as 
Zsigmondy1  has  done  in  the  case  of  silicic  acid  gel,  and  combining  this 
value  with  that  number  representing  the  internal  volume  of  a  definite 
mass  of  the  substance.  This,  at  least,  would  give  an  idea  of  the  internal 
surface.  Yet,  granting  that  a  fairly  accurate  estimation  were  possible,  it 
certainly  must  be  admitted  that  forces  other  than  residual  valence  come 
into  play,  especially  so  when  the  pores  themselves  approach  the  vicinity 
of  molecular  dimensions.  This  fact  Langmuir  recognizes  and  suggests 
that  true  adsorption  should  deal  with  plane  or  smooth  surfaces  only.  It 
is  thus  evident  that  the  observations  made  in  this  investigation  cannot 
be  expressed  by  the  Langmuir  equation  in  its  present  form. 

The  simplest  and  most  widely  used  adsorption  equation  is  that  pro- 
posed by  Freundlich.  This  is  purely  an  empirical  relation,  but  one  that 
is  very  elastic  and  easy  of  manipulation.  If  %  is  the  amount  adsorbed,  m 
the  mass  of  the  gel,  p  the  pressure  of  the  gas,  a  and  i/n  constants,  the 
equation  is  expressed  as  follows: 

x/m  =  ap  i/n, 
or  writing  in  the  logarithmic  form, 

log  x/m  =  log  a  +  i/n  log  p. 

This  is  an  equation  of  a  straight  line  and  hence  the  constants  a  and  i/n 
are  easily  interpreted — a  being  the  amount  adsorbed  when  the  pressure 
is  unity,  and  i/n  representing  the  slope  of  the  line.  It  is  readily  seen 
that  the  constants  change  with  a  change  from  one  temperature  to  an- 
other. So  in  order  to  make  a  perfect  general  equation  this  change  must 
be  expressed. 

An  inspection  of  Figs.  2  and  3  will  show  that  the  results  obtained  with 
silica  gel  and  sulfur  dioxide  are  very  well  represented  by  the  Freundlich 
equation.     For  this  reason  the  constants  a  and  i/n  have  been  given  in 
1  Loc.  cit. 


29 

the  tables  containing  the  data.  The  value  of  i/n  given  at  the  head  of 
each  experiment  was  obtained  by  the  method  of  mean  errors  and  from 
that  the  value  of  a  was  found  by  substitution  in  one  of  the  equations. 
This  value  of  a,  you  will  notice,  corresponds  very  closely  to  what  would 
be  read  from  the  graph  shown  in  Fig.  3. 

A  very  exhaustive  treatment  of  this  equation  and  its  relation  to  tempera- 
ture is  given  by  Freundlich1  and  for  this  reason  it  is  not  necessary  to  carry 
through  the  somewhat  extended  proof  for  the  validity  of  the  general 
equation,  which  takes  into  consideration  all  the  variables — pressure, 
temperature  and  amount  adsorbed.  It  has  the  following  form: 
log  (x/m)t  =  log  (*/m)0  —  (z  —  y  log  p)t, 

d  log  a  d  i  In      —, 

where  z  =  —  — -7^—  and  y  =  .     These  values  y  and  z  should  be 

constants  and  although  the  experimental  results  do  not  strictly  bear  this 
out,  yet  there  is  sufficient  constancy  to  make  calculations  that  give  good 
approximate  agreement.  Table  III  gives  the  observed  values  and  those 
calculated  from  the  equation  above,  using  the  observations  made  in  Expts. 
XVIII  and  XIX.  For  this  particular  sample  of  gel  z  =  — 0.0146  and 
y  =  0.0035,  values  obtained  by  taking  a  weighted  mean  of  these  differ- 
entials actually  observed  at  the  temperatures  from  o°  to  100°. 

TABLE  HI. 
Expt.  XVIII  (c)  40°.  Expt.  XIX  (c)  80°. 

P.  X/M  obs.  X/Mcalc.  P.  X/M  obs.          X/M  calc. 

9.44  7.50  ...  47.00  4.48  6.20 

31.37  16.50  14-45  192.19  16.73  16.75 

64.77  25.25  22.62  224.73  18.66  18.70 

no.oo  33.66  30.73  407.88  27.70  28.99 

169.13  41.88  39.01  575-32  34-17  37-05 

299.78  55-54  53-91  671.95  37.41  41.20 

448.60  67.17  67.99 

567.52  75-07  77-81 

692.20  82.16  84.36 

The  objectionable  feature  of  the  Freundlich  equation,  as  well  as  to  most 
all  other  adsorption  formulas  yet  proposed,  is  that  isotherms  at  many 
different  temperatures  have  to  be  made  in  order  to  obtain  the  proper  value 
of  the  constants  to  be  used  for  adsorption  values  at  any  pressure  and  at 
any  temperature.  There  is  no  way  of  predicting  or  even  roughly  ap- 
proximating what  the  adsorption  would  be  at  a  temperature,  say  40°, 
knowing  the  adsorption  at  some  other  temperature,  say  o°.  This  means 
that  a  very  large  number  of  experiments  must  be  made  on  every  system 
before  it  can  be  properly  disposed  of  and  cataloged.  This  point  will  be 
taken  up  more  fully  in  the  latter  part  of  the  paper. 

The  accuracy  of  the  measurements  and  the  ease  with  which  they  can 
1  Freundlich,  "Kapillarchemie,"  p.  101. 


30 

be  reproduced  is  clearly  shown  by  Expts.  XV,  XVI  and  XVII,  which 
were  carried  out  on  different  dates  with  2 . 1422  g.,  i .  5440  g.  and  2 . 224  g. 
of  gel,  respectively.  The  values  of  X/M  at  equal  pressures  were  cal- 
culated by  the  aid  of  the  Freundlich  equation.  These  calculations  are 
found  in  Table  IV. 

TABLE  IV. 
X/M  Calculated  from  i/n  and  a  Values. 

P.  Expt.  XV.  Expt.  XVI.  Expt.  XXIV. 

(Cm.).  June  26.  July  4.  Sept.  16. 

5  58.45  56.70  59.36 

10  78.68  77-33  79-86 

15  93.63  92.73  94-97 

20  105.91  105.48  107.40 

25  113.90  116.55  118.16 

30  126.03  126.47  127.75 

35  I34-64  I35-5I  139.64 

40  142.58  143.86  144.48 

45  149-93  151-66  151.95 

50  159-58  158.98  158.96 

55  163-44  165.91  165.57 

65  175-57  178.80  177-84 

In  Fig.  3  we  have  plotted  log  X/M  against  log  p.  If  the  equation  held 
absolutely  we  would  have  a  system  of  straight  nearly  parallel  lines.  This 
is  not  strictly  true.  There  are  deviations  in  both  directions,  but  more 
noticeably  so  with  those  isotherms  carried  out  at  the  extreme  tempera- 
tures. This  bending  is  concave  towards  the  #-axis,  and  for  high  tempera- 
tures takes  place  at  the  extreme  left,  while  at  the  lower  temperatures 
it  occurs  at  the  extreme  right.  The  first  case  is  probably  due  to  the  slight 
pressure  developed  by  the  adsorbed  air  released  on  the  introduction  of 
the  first  amount  of  sulfur  dioxide.  This  pressure,  although  extremely 
small  in  itself,  is,  in  proportion  to  the  pressure  of  sulfur  dioxide  realtively 
large  at  this  part  of  the  curve  and  hence  would  produce  a  noticeable 
effect.  More  will  be  said  later  in  regard  to  this  point.  The  bending  in 
the  case  of  the  lower  temperatures  is  easily  accounted  for.  In  that  region 
the  vapor  pressure  of  the  liquid  is  approached  and  deviations  would  not 
be  surprising  but  expected.  Others1  have  shown  that  where  p/p0  ap- 
proaches unity  the  Freundlich  equation  is  not  applicable. 

The  mere  fact  that  a  chemically  inert  substance  like  silica  gel  is  found 
exhibiting  such  marked  adsorptive  properties  is  sufficient  in  itself  to  indi- 
cate that  the  cause  of  adsorption  does  not  lie  in  the  interaction  of  adsorbent 
and  adsorbed  substance.  In  making  the  above  statement  we  do  not  mean 
to  say  that  it  covers  all  the  cases  of  gas  or  vapor  adsorption,  for  the  fact 
of  specific  gas  adsorbents  would  tend  to  disprove  it,  e.  g.,  palladium  for 
hydrogen.  Perhaps  it  would  be  better  to  confine  ourselves  to  the  ad- 
1  Titoff,  Z.  physik.  Chem.,  74,  641  (1910);  I/.  B.  Richardson,  /.  Am.  Chem.  Soc.,  39, 
1828  (1917). 


sorption  of  vapors,  although  it  will  be  seen  that  our  analysis  permits  the 
extensions  to  regions  that  are  ordinarily  considered  as  gaseous.  As  an 
approximate  line  of  division  we  might  select  the  critical  temperature  and 
confine  ourselves  to  a  discussion  of  adsorption  occurring  below  this  tem- 
perature. It  cannot  be  too  strongly  emphasized  that  we  are  dealing  with 
phenomena  that  exhibit  adsorption  to  a  marked  degree,  and  are  not  mani- 
festations of  layers  of  a  few  molecules  deep. 

It  is  our  belief  that  the  adsorption  of  gases  or  vapors,  let  us  say  at 
all  temperatures  below  the  critical  temperature,  may  be  predicted  from 
a  knowledge  of  the  physical  constants  of  the-gas  or  vapor  alone.  Further- 
more, the  role  of  the  adsorbent  is  simply  that  of  a  porous  body,  its  chemical 
nature  being  a  matter  of  indifference.  (Cases  of  obvious  chemical  affinity 
are  of  course  excluded.)  Adsorbents  differ  in  the  extent  of  their  total 
internal  volume  and  also  in  the  dimensions  of  the  spaces,  called  pores 
for  simplicity,  that  make  up  the  internal  volume.  It  is  conceivable  that 
2  adsorbents  may  possess  the  same  internal  volume  but  show  marked  differ- 
ences in  the  adsorption  of  the  same  vapor  due  to  differences  in  the  distribu- 
tion of  the  pore  sizes. 

If  this  is  true  the  form  of  the  adsorption  curve  expresses  the  distribu- 
tion of  the  internal  volume  as  a  function  of  the  dimensions  of  the  pores. 
An  attempt  was  made  to  express  this  relation  in  terms  of  the  Maxwell 
distribution  law,  but  a  moment's  reflection  will  convince  one  that  there 
is  no  reason  to  expect  the  pore  sizes  to  be  distributed  according  to  the 
laws  of  probability.  The  pores  in  the  silica  gel  exist  as  the  result  of  the 
juxtaposition  of  colloidal  particles  which  are  approximately  all  of  equal 
dimensions  and  are,  therefore,  probably  V-shape  in  cross  section,  or  at 
any  rate  may  be  designated  as  tapering. 

It  is  at  once  evident  that  if  the  adsorption  curve  simply  shows  the  man- 
ner in  which  the  various  sized  pores  are  distributed  that  go  to  make  up 
the  internal  volume  of  the  adsorbent,  then,  instead  of  seeking  a  relation 
between  weight  of  adsorbed  gas  and  the  equilibrium  pressure  we  should 
at  once  turn  to  the  volume  occupied  by  the  adsorbed  gas.  As  a  matter 
of  fact,  if  we  express  our  isotherms  of  sulfur  dioxide  adsorption  with  volume 
of  liquid  sulfur  dioxide  as  ordinates  instead  of  weight,  the  curves  are 
brought  closer  together.  Our  next  consideration  is,  of  course,  to  express 
the  abscissas  of  our  isotherms  not  as  simple  equilibrium  pressures  but  as 
corresponding  condensation  pressures. 

It  has  long  been  known  that  the  properties  which  determine  the  ease 
of  condensation  of  a  gas  or  vapor  are  closely  connected  with  the  physical 
constants  of  the  gas  or  vapor  which  are  of  importance  in  determining 
the  magnitude  of  the  adsorption.  It  is  well  known  that  condensations 
of  vapors  occur  with  greater  ease  in  capillary  tubes  than  on  a  level  sur- 
face, provided  the  liquid  wets  the  capillary  wall.  This  phenomenon 


has  been  long  studied  and  the  lowering  of  the  vapor  pressure  of  a  liquid 
in  a  capillary  in  terms  of  the  ordinary  vapor  pressure  of  the  liquid  P0 
is  given  by  the  following  relation: 


where  a  is  the  surface  tension,  d  the  density  of  the  saturated  vapor,  D 
the  density  of  the  liquid  and  r  the  radius  of  the  capillary.  With  the  aid 
of  this  relationship  we  can  readily  derive  the  fact  that  the  radius  of  the 
tube  must  be  very  small  in  order  to  have  an  appreciable  effect  on  the 
vapor  pressure  of  the  liquid  inside.  It  is  not  until  we  get  to  tubes  of  less 
than  o.ooi  mm.  in  diameter  that  we  begin  to  affect  the  vapor  pressure. 
From  this  it  is  clear  that  if  we  wish  to  account  for  the  marked  lowering 
of  the  vapor  pressure  in  the  case  of  adsorption,  pores  approaching  molecu- 
lar magnitude  must  be  assumed.  It  is  our  feeling  that  such  a  wide  ex- 
trapolation of  the  above  formula  is  not  justified  and  in  the  present  anal- 
ysis we  shall  not  consider  the  question  of  absolute  diameter  of  pores. 

If  we  wish  to  compare  the  adsorption  of  a  particular  adsorbent  for  a  gas 
or  vapor  at  various  temperatures,  it  is  evident  that  the  comparison  must 
not  be  made  at  the  same  pressure,  but  rather  at  some  corresponding  pres- 
sure. As  suggested  by  Williams  and  Donnan1  the  value  of  p/p0  may  be 
selected  for  this  purpose  (p0  is  the  vapor  pressure  of  the  condensed  vapor) . 

In  Fig.  8  we  have  plotted  the  logarithms  of  the  volumes  of  condensed 
sulfur  dioxide  (obtained  by  dividing  the  weight  of  sulfur  dioxide  by  the 
density  of  liquid  sulfur  dioxide  at  the  corresponding  temperature)  as 
ordinates  against  the  values  of  logarithm  p/p0  as  abscissas.  It  will 
be  noted  that  greater 
volumes  are  taken  up 
at  lower  temperatures 
at  the  same  corre- 
sponding pressures. 
Furthermore,  it  is  to 
be  noted  that  all  the 
adsorption  isotherms 
are  brought  much 
closer  together.  When 
P/Po  equals  unity  the 
same  volume  of  sulfur  dioxide  is  taken  up  at  all  temperatures.  At  the 
higher  temperature  we  were  unable  to  work  with  pressures  sufficiently 
great  to  enable  us  to  realize  the  value  of  unity  for  p/pot  however,  the 
slope  of  the  log  curves  is  such  as  to  bring  all  curves  together  at  the 
point  p/po  =  i. 

An  approximate  idea  of  exactly  what  this  volume  is  may  be  grasped  by 
1  Williams  and  Donnan,  Trans.  Faraday  Soc.,  10  (1914). 


33 


reference  to  Fig.  9.     Here  are  plotted  on  a  larger  scale  the  results  ob- 
tained at  the  lower  temperatures,  in  fact  those  temperatures  where  the 

saturation  point  was 
reached.  This  point 
is  easily  fixed  by  the 
very  sharp  break  in 
the  curve.  Introduc- 
ing density  correction, 
these  values  become 
almost  identical. 
Table  V  gives  these 
results,  corrected  and 
uncorrected,  as  well 
as  the  saturation 
value  of  the  isotherm 
at  o°  calculated  from 
the  adsorption  equa- 
tion. The  accuracy 
of  the  Freundlich  equation  does  not  permit  calculation  of  the  saturation 
points  at  the  higher  temperatures  as  a  wide  deviation  would  be  expected. 

TABLE  V. 

Temperature — 80° 

Vol.  gas  phase,  cc 232 

Vol.  liquid  phase,  cc.  (or  internal  vol.  of  gel) .     o .  4073 


—54" 
228 
0.4168 


— 34-0' 
216 


0.4039 


209 
0.4167 


Similar  results  with  silica  gel  were  obtained  by  Bachmann.1  This  in- 
vestigator showed  that  with  the  same  sample  of  gel  at  the  saturation 
pressure,  that  is  the  vapor  pressure  of  the  liquid  at  that  temperature, 
the  same  volume  of  different  liquids  was  taken  up.  Some  experiments 
were  carried  out  in  which  the  liquid  was  introduced  through  the  gas 
phase;  others  where  the  gel  was  introduced  directly  into  the  liquid.  In 
this  latter  case  the  surface  was  carefully  wiped  with  filter  paper  and  possi- 
ble errors  from  this  source  minimized.  The  author  states  that  no  correc- 
tion for  contraction  or  other  volume  change  resulting  from  possible  forces 
acting  within  the  gel  structure  was  considered  in  the  calculation.  A 
few  determinations  are  given.2 

18°.  Sample  2.  0.3572  g.  gel. 


Wt.  absorbed. 
Liquid.  G. 

H20 0.2276 

C«H6 0.8791 

C2H2Br4 0.6720 


Vol.  per  g.  of  gel. 
Cc. 

0.6210 
0.6270 
0.6210 


1  W.  Bachmann,  Z.  anorg.  Chem.,  79,  202  (1913). 

2  Other  gel  samples  gave  consistent  although  different  values  from  the  above, 
e.  g.,  Sample  5 — vol.  =  0.3621  cc.;  loc.  cit. 


34 

The  absolute  value  is  not  in  agreement  with  that  found  in  this  investiga- 
tion, but  it  must  be  remembered  that  the  experimental  method  as  well 
as  the  gel  sample  itself  was  different.  The  main  point  is  that  with  the 
same  gel  sample  there  is  an  equal  volume  of  the  liquid  adsorbed,  no  matter 
what  the  liquid  or  what  the  temperature. 

Up  to  this  point  we  have  considered  the  lowering  of  the  vapor  pressure 
from  the  simple  standpoint  as  being  due  to  the  rise  in  a  capillary  tube. 
Clearly,  in  our  case  the  effect  is  not  due  to  a  difference  in  level,  nor  is  it 
certain  that  we  are  dealing  with  tubes  opened  at  both  ends.  For  our 
purpose  it  is  better  to  consider  the  lowering  of  the  vapor  pressure  of  the 
liquid  in  a  pore  as  due  to  a  negative  tension  exerted  on  the  liquid  around 
the  meniscus.  Thus  this  pull  has  its  origin  in  the  tendency  of  films  which 
wet  the  walls  to  contract  so  as  to  expose  as  little  of  surface  as  possible. 
Looking  at  the  adsorption  of  vapors  in  this  light,  it  is  seen  that  the  con- 
densed vapor  is  under  a  tension  rather  than  a  pressure.  Furthermore, 
it  is  a  simple  matter  to  calculate  the  magnitude  of  this  negative  pressure. 
Using  the  well-known  Gibbs  relation, 


where  dp  =  change  in  the  vapor  pressure,  dP  =  change  in  the  hydrostatic 
pressure,  V  =  volume  of  the  condensed  phase,  and  v  —  volume  of  the 
gas  phase,  expressing  the  variation  of  vapor  pressure  with  the  hydro- 
static pressure,  we  can  calculate  that  liquid  sulfur  dioxide  at  30°, 
having  a  vapor  pressure  of  3496  mm.,  when  in  a  capillary  tube 
under  a  vapor  pressure  of  9.55  mm.,  is  subject  to  a  tension  of  about 
530  atmospheres.  When  the  pressure  over  the  condensed  liquid  sulfur 
dioxide  has  risen  to  706  mm.  by  the  above  relationship  it  can  be  shown 
that  the  negative  pressure  has  fallen  to  420  atmospheres.  It  is  evident 
that  we  are  in  a  position  to  calculate  the  negative  pressure  on  any  liquid 
provided  we  know  the  lowering  of  the  vapor  pressure,  and  the  density 
of  the  condensed  phase.  (It  is  assumed  that  the  vapor  obeys  the  gas 
laws.) 

If  the  liquid  is  in  a  closed  tube  this  pull  must  occasion  a  dilation  of 
the  same  to  an  extent  that  is  proportional  to  the  compressibility  of  the 
liquid.  Worthington1  has  stated  that  the  volume  changes  caused  by 
negative  pressure  may  be  calculated  with  the  aid  of  the  compressibility 
coefficient.  Unfortunately,  we  have  no  direct  measurements  of  the 
compressibility  of  liquid  sulfur  dioxide  and  are,  therefore,  unable  to 
evaluate  quantitatively  the  volume  change.  It  is  known  that  in  some 
cases2  the  relation 

&     */*  I/" 

Lj{f  SB    jfx 

1  Worthington,  Trans.  Roy.  Soc.  (London),  iSsA,  355  (1892). 

2  Richards,  J.  Am.  Chem.  Soc.,  40,  59  (1919). 


35 


holds  good,  but  it  has  only  been  tested  over  a  narrow  range  of  a  and  many 
exceptions  have  been  noted.  We  can,  however,  say  that  liquids  of  high 
surface  tension  have  smaller  compressibilities  than  liquids  of  low  surface 
tension. 

Here  we  have  a  possible  explanation  for  the  fact  that  the  volume  of 
sulfur  dioxide  at  corresponding  pressures  are  smaller  at  high  than  at  low 
temperature.  At  the  higher  temperature  the  condensed  phase  is  more 
compressible,  cr,  being  smaller,  and  in  addition  the  negative  pressure  is 
greater.  In  other  words,  we  do  not  know  the  actual  density  of  the  con- 
densed phase  in  the  gel,  but  in  all  cases  it  is  lower  than  the  normal  density 
which  it  approaches  when  p/po  =  i. 


Expt.  XII 

Expt.  XXIII. 

100°. 

30°. 

<r  =  9.25,  D 

=  I.  Ill,  p<. 

>   =  2114.3  cm.     a  = 

22.75,  D 

=    L3556,  Po 

=  349.6  cm. 

log  V. 

log  P/po. 

log  PV/PO. 

log  V. 

103  P/Po. 

log  pff/po. 

2.49183 

5.03643 

7.00257 

2-39811 

3.33415 

2.69113 

2  .66113 

2.27363 

7.23977 

z  .  73466 

2.06453 

7.42151 

2-75343 

2.41060 

7.37674 

i.  88680 

2.37194 

1.72892 

2  .  80302 

2.50189 

i  .  46803 

•00355 

2  .  60706 

7  .  96404 

.  11206 

2.83983 

0.09681 

.21380 

7.067II 

0.35698 

.28652 

7.22883 

0.58581 

.32130 

7.3O30I 

0.65999 

Expt.  XVIII. 

40°. 

tr  =  21.0, 

D  =  1.3111,  po 

=  471.2  cm 

log  V. 

log  p/po. 

log  Pff/po. 

2.21395 

3-30176 

2.62398 

2.55637 

3-82330 

7.14552 

2.74H5 

2.13816 

7.46038 

2.86600 

2.36818 

7  .  69040 

2"  .  96090 

2.55501 

1.87723 

7.08350 

2.80359 

0.12581 

7.16607 

2.97865 

0.30087 

7.21436 

7.08077 

0,40299 

1-25355 

7.16702 

0.48944 

Expt.  XXVI. 

-54°. 

=  39-0  1 

?    =    1.565,  Po    = 

88.3  mm. 

log  V. 

log  P/po. 

log  pff/po. 

2.75603 

3.65610 

7.24716 

7.16510 

5.67313 

0.26419 

7.30525 

7.04748 

0.63854 

1.41792 

7.29832 

0.88938 

7.49483 

1-49337 

1.08443 

1.57984 

i  -  T0435 

1-29541 

Expt.  XIX. 
80°. 

o-  =  13.1,  D  =  1.192,  po  =  1368  cm. 

log  V.                    log  p/po.  log  Pff/po 

2.60374                2.14764  ".26491 

2.65H5                 2.21557  "-33284 

2.82272     2.47444  .59171 

2.91388     2.62382  ".74109 

2.95323     2.69124  ".80851 


Expt.  XXV. 

—80°. 
1.6295,  <r  —  44-5,  P°  =  8.8  mm. 


log  V. 

2.94694 
•23931 
.41420 
.50416 
•55975 
.60585 


log  p/po  . 
2.16946 
2.8l895 
7.27300 
7.53982 
7.71728 
1.97460 


13J  Pff/Po. 
7.81782 

0.46731 
0.92136 
I. I88l8 

1.36564 
I .62296 


Expt.  XXIX 

Expt.  XXIV. 

-34-4°. 

0°. 

r   =  36.2,  D 

=  1.5302,  p° 

=  232.6  mm.     <7  =  28.5,  D  =  1.435,  p°  = 

1  1  6.2  cm. 

lo;  V 

lo  ;  P/po. 

log  pff/po.                        log  V.                log  P/PO. 

log  Pa/Po. 

2.99858 

2.25870 

T.8I74I                       2.79757             3-95307 

^.40755 

.18293 

2.77484 

0-33355                       2.99515             2.44763 

I  .  9O2  I  I 

.30592 

1.09609 

o  .  65480 

".13180         2.76370 

o.  21818 

.40903 

T-33297 

0.89168 

".20767         2.95742 

0.4II90 

.48289 

1.49938 

I  .  04809 

".25764 

".10226 

0.55674 

•55521 

I.700I6 

1.25887 

".31482 

".21489 

0.66937 

.58669 

T.94639 

1.50510 

.36637 

".32496 

0-77944 

.  40847 

".42696 

0.88244 

.44278 

"•49994 

0.95442 

.49499 

"  .  60484 

1.05932 

.53977        1 

".70266 

I.I57I4 

.56726 

".79221 

I  .  24669 

As  an  empirical  relationship,  the  result  of  dividing  the  volume  of  the 
condensed  sulfur  dioxide  by  the  value  of  the  surface  tension  raised  to  a 
fractional  power  was  tried.  Qualitatively,  this  produces  a  correction  in 
the  right  direction.  In  order  to  take  into  consideration  the  constant 
that  connects  the  value  of  the  surface  tension  with  the  change  of  volume, 
we  have  thrown  our  relation  into  the  following  form : 

V    =  k(P/P0)l/n 


l/n 


which,  for  calculation  purposes,  can  be  arranged  thus 


V  =  Ki~ 


l/n 


assuming  that  the  same  value  of  i/n  holds  both  for  p/p0  and  <7. 

The  preceding  tables  show  the  value  of  V,  p/po  and  Pa/p0 ,  all  expressed 
as  logs  for  convenience  in  plotting.  Fig.  10  shows  the  contents  of 
these  tables  when  plotted  with  log  V  as  abscissas  and  log  of  Pa/pQ  as 
ordinates. 


From  this  logarithm  curve  the  value  of  the  constant  i/n  and  k  are 
found  to  be  0.447  and  0.1038.     In  this  case  k  has  been  taken  as  that 


37 

p 
volume  where  —  =  i  and  i/n  has  its  usual  significance — the  slope  of 

Po 

the  curve.     Hence  our  adsorption  equation  for  the  system  silica  gel — 
sulfur  dioxide  would  be, 


V  =  0.1038  (  —  )  0.447, 

where  V  is  expressed  in  cubic  centimeters,  <rin  dynes/cm.,  and  p  and  p0 
in  the  same  unit  of  pressure.  The  close  agreement  is  very  striking  and 
is  strong  evidence  of  our  claim  that  the  volume  occupied  by  the  adsorbed 
vapor  is  the  same  at  the  same  value  of  the  corresponding  pressure  p/p0  . 

Summary. 

1.  The  adsorption  of  sulfur  dioxide  by  silica  gel  was  measured  at  various 
temperatures  between  —80°  and  +100°. 

2.  The  effect  of  the  water  content  of  the  silica  gel  was  studied.     Maxi- 
mum adsorption  was  shown  by  gels  containing  about  7%  water. 

3.  The  adsorption  was  shown  to  be  reversible  in  the  absence  of  air.     In 
the  presence  of  small  amounts  of  air  the  rate  of  adsorption  was  greatly 
decreased  and  adsorption  and  desorption  were  irreversible. 

4.  The  empirical  equation  of  Freundlich  was  found  to  hold  over  almost 
the  entire  range  studied  —  exceptions  being  at  these  points  where  the 
saturation  pressure  was  approached. 

5.  The  equation 


is  found  to  hold,  where  V  =  volume  of  condensed  phase  uncorrected,  cr  the 
surface  tension,  p  the  pressure  of  the  gas  phase,  p0  the  vapor  pressure  of 
the  liquid,  k  and  i/n  constants  dependent  upon  the  physical  properties  of 
the  adsorbent. 


BIOGRAPHY. 

John  McGavack,  Jr.,  was  born  February  10,  1893,  at  Waterford,  Vir- 
ginia. He  received  his  early  education  in  the  schools  of  that  section  and 
entered  Hampden-Sidney  College,  Hampden-Sidney,  Virginia,  in  the  fall 
of  1910,  from  which  college  he  was  graduated  with  the  degree  of  Bachelor 
of  Arts  in  1913.  From  1913-16  he  was  instructor  in  the  Charles  Town 
High  School,  Charles  Town,  West  Virginia.  In  the  fall  of  1916  he  entered 
the  graduate  department  of  the  Johns  Hopkins  University,  majoring  in 
Chemistry  with  Physical  Chemistry  and  Mathematics  as  his  first  and 
second  subordinate  subjects,  respectively.  From  June,  1917,  until 
December,  1918,  he  was  connected  with  the  war  investigations  carried 
out  in  this  laboratory  first  as  an  employee  of  the  Bureau  of  Mines  and  then 
as  a  member  of  the  Chemical  Warfare  Service.  He  received  a  fellowship 
in  Chemistry  for  his  last  half  term  of  1918-19  and  was  Research  Assistant 
in  Physical  Chemistry  during  the  session  1919-20. 


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